# What drives the growth of black holes: a decade of progress

D. Alexander, R. Hickox, J. Aird, F. Combes, T. Costa, M. Habouzit, C. Harrison, R. Leng, L. Morabito, S. Uckelman, et al.

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# What drives the growth of black holes: a decade of progress

D. M. Alexander<sup>a</sup>, R. C. Hickox<sup>b</sup>, J. Aird<sup>c</sup>, F. Combes<sup>d</sup>, T. Costa<sup>e</sup>, M. Habouzit<sup>f,g</sup>, C. M. Harrison<sup>e</sup>, R. I. Leng<sup>h</sup>, L. K. Morabito<sup>a,i</sup>, S. L. Uckelman<sup>j</sup>, P. Vickers<sup>j</sup>

<sup>a</sup>Centre for Extragalactic Astronomy, Department of Physics. Durham University, South Road, Durham, DH1 3LE, UK

<sup>b</sup>Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA

<sup>c</sup>Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK

<sup>d</sup>Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne University, Paris, F-75014, France

<sup>e</sup>School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK

<sup>f</sup>Department of Astronomy, University of Geneva, Ch. d'Ecogia 16, Versoix, CH-1290, Switzerland

<sup>g</sup>Max-Planck-Institut für Astronomie, Königstuhl 17, Heidelberg, D-69117, Germany

<sup>h</sup>School of Social Science and Political Science, University of Edinburgh, 15a George Square, Edinburgh, EH8 9LD, UK

<sup>i</sup>Institute of Computational Cosmology, Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK

<sup>j</sup>Department of Philosophy, Durham University, 50 Old Elvet, Durham, DH1 3HN, UK

## Abstract

The last decade has witnessed significant progress in our understanding of the growth of super-massive black holes (SMBHs). It is now clear that an Active Galactic Nucleus (AGN: the observed manifestation of a growing SMBH) is an “event” within the broader lifecycle of a galaxy, which can significantly influence the shape and evolution of the galaxy itself. Our view of the obscuring medium that affects the observed properties of an AGN has also undergone a revolution, and we now have a more physical understanding of the connection between the fuelling of (and feedback from) the SMBH and the broader host-galaxy and larger-scale environment. We have a greater understanding of the physics of SMBH accretion, can identify AGNs out to z = 8–10 witnessing the very earliest phases of SMBH growth, and have a more complete census of AGN activity than ever before. This great progress has been enabled by new innovative facilities, an ever-increasing quantity of multi-wavelength data, the exploitation and development of new techniques, and greater community-wide engagement. In this article we review our understanding of AGNs and the growth of SMBHs, providing an update of the earlier Alexander and Hickox (2012) review. Using citation-network analyses we also show where this review fits within the broader black-hole research literature and, adopting the previous article as a snapshot of the field over a decade ago, identify the drivers that have enabled the greatest scientific progress.

**Keywords:** black holes, accretion, active galactic nuclei, quasars, feedback, galaxies

## 1. Introduction

Once considered exceptional events occuring within the centres of galaxies, Active Galactic Nuclei (AGNs) are now understood to be an integral component in the formation and evolution of galaxies. AGN activity is driven by the accretion of gas onto a Super-Massive Black Hole (SMBH; Shakura and Sunyaev, 1973; Soltan, 1982; Rees, 1984), which gives rise to a broad multi-wavelength spectral energy distribution (SED) extending from radio to X-ray (and gamma rays in some cases; e.g., Elvis et al., 1994; Padovani et al., 2017). During this mass-accretion phase, a substantial fraction of the mass and energy can also be injected into the broader host-galaxy environment (and even beyond) in the form of radiation, accretion-disk driven winds, and energetic jets, allowing the AGN to influence the growth of the galaxy (e.g., “AGN feedback”; Fabian, 2012; King and Pounds, 2015; Harrison, 2017).

The majority of the gas that gives rise to this AGN activity is, at least initially, distributed over the broader host-galaxy environment, which is orders of magnitude larger than the SMBH. For example, for a non-rotating $10^8 M_{\odot}$ SMBH, the event horizon ($R_S$; $\approx 3 \times 10^{-5}$ pc) and the radius of gravitational influence

($R_{\text{Rol}}$; $\approx 10$ pc) are $\approx 9$ and $\approx 3$ orders of magnitude smaller than the host galaxy, respectively.<fnref n="1" /> To overcome this vast difference in size scale the gas needs to lose angular momentum through gravitational torques from stars either within the galaxy or external to the galaxy (e.g., from galaxy mergers and interactions; Jogee, 2006; Hopkins and Quataert, 2010). Once the gas reaches $R_{\text{Rol}}$, the SMBH dominates the gravitational potential and, along with radiation and magnetic pressure, gas inflow and outflow, defines the key features of the standard AGN model (e.g., the dusty “torus”; broad-line region; accretion disk; Netzer, 2015; Ramos Almeida and Ricci, 2017; Hickox and Alexander, 2018). See Fig. 1 for a schematic representation of the key components of an AGN within the host galaxy environment and Fig. 2 for colour-coded broad-band SEDs of these components and the radio-gamma ray waveband definitions.

Despite this angular-momentum challenge, we know that AGN “events” occur regularly in many (potentially all) galaxies.

<footnote n="1">¹The radius of the event horizon of a non-rotating SMBH is calculated as $R_S = 2GM_{\text{BH}}/c^2$ (i.e., the Schwarzschild radius) and the radius of the gravitational influence of a SMBH is calculated as $R_{\text{Rol}} = GM_{\text{BH}}/\sigma^2$, where $M_{\text{BH}}$ is the SMBH mass and $\sigma$ is the velocity dispersion of the host galaxy.</footnote>
<figure><img src="image_2.png" /><figcaption>Figure 1: (a) Simple schematic representation of an AGN within the broader host-galaxy and halo environment over (logarithmic) milli-parsec to mega-parsec scales in the horizontal plane; note the schematics are not scaled in the vertical plane. Key regions are highlighted including those under the gravitational influence of the SMBH (the AGN: accretion disk, X-ray "corona", potential winds/jets launched from the accretion disk, broad-line region (BLR), and the dusty molecular "torus"), the host galaxy, and halo environment. The AGN is aligned with the host galaxy for presentation purposes; in reality since it is under the gravitational influence of the SMBH, it can be randomly orientated with respect to the host galaxy. The general schematic style is inspired by Ramos Almeida and Ricci (2017) but using a colour scheme to match the SED components in Fig. 2. (b) Multiphase gas cross-section simulation over (logarithmic) milli-parsec to mega-parsec scales. The *r* coordinate is defined from the centre of the SMBH and the cross section is displayed to cover the range -*r* to +*r* in the *z* direction. Colours denote the different gas-phase temperatures: T < 10³ K (green), 10³ < T < 10⁴ K (yellow), 10⁴ < T < 10⁵ K (magenta), 10⁵ < T < 10⁶ K (purple), T > 10⁶ K (cyan). The various regions have been highlighted: IGM (intergalactic medium), CGM (circumgalactic medium), ISM (interstellar medium), central kpc region of the galaxy, and the outer and inner regions of the accretion disk. See Harrison and Ramos Almeida (2024) for a complementary schematic representation of the gas conditions over the same size scales. Source: panel b adapted from Fig. 9 of Hopkins et al. (2024).</figcaption></figure>

Key evidence for this view is (1) the identification (or inference) of a dormant SMBH at the centres of most galaxies, and (2) the remarkable relationship found between the mass of the SMBH and the galaxy bulge in nearby galaxies, which provides both the archaeological evidence for past phases of AGN activity and infers a connection between the growth of the SMBH and the host galaxy (e.g., Alexander and Hickox, 2012; Kor mendy and Ho, 2013; Heckman and Best, 2014).

The aims of this article are twofold. The first, and primary, objective is to review our understanding of AGN to address the very broad question: what drives the growth of black holes? This article provides an update of our previous Alexander and Hickox (2012) review (hereafter AH12) and describes the exciting scientific discoveries and progress made over the last decade. The second objective is to investigate the drivers of this scientific progress, using the AH12 review as a “snapshot” of the research field from over a decade ago. This second objective provides interesting insight on how science progresses within a rapidly advancing research discipline. To enable the closest tracking of scientific progress in this article, we have used the same scientific sections as those adopted in AH12.

From a very broad perspective, the last decade has witnessed major discoveries in the field of AGN and black holes. The Event Horizon Telescope (EHT; Event Horizon Telescope Collaboration et al., 2019a) has used a global (world wide) very

long baseline interferometer to take the first “direct” image of a SMBH at millimetre wavelengths in two systems: the nearby massive galaxy M87 and Sgr A* at the centre of our Milky Way galaxy (Event Horizon Telescope Collaboration et al., 2019b, 2022). The SMBHs are identified from the silhouette or shadow due to them lying in the foreground of synchrotron emission produced in the vicinity of the SMBH. From the shape and size of the SMBH shadow, key fundamental measurements can be inferred (e.g., the mass and spin of the SMBH), even testing general relativity, and complementing other on-going experiments (see Genzel et al., 2024, for a review).

The whole of physics, not just astronomy, has been transformed by the first significant detections of gravitational waves (LIGO Scientific Collaboration et al., 2015; Abbott et al., 2016). The gravitational waves are produced from the energy losses of two compact objects (i.e., black holes or neutron stars) in a tightly bound orbit and peak just prior to the objects coalescing (e.g., Blanchet, 2014). The current gravitational-wave observatories are most sensitive to stellar-mass objects and have detected gravitational waves from ≈ 100 merging systems with masses up-to ≈ 150 $M_{\odot}$ (Abbott et al., 2020, 2023). Pulsar timing arrays (PTAs) are providing the first gravitational-wave constraints from SMBH mergers and have identified a gravitational-wave background believed to be produced by merging SMBHs (e.g. EPTA Collaboration et al.,
<figure><img src="image_3.png" /><figcaption>Figure 2: Broad-band SED of an unobscured AGN (black curve) across the broad gamma ray–radio waveband: the frequency and wavelength ranges for individual wavebands are highlighted. Individual continuum-emitting components are coloured and labelled including emission from a star-forming component (grey curve) and the brighter radio emission from a radio-loud AGN (labelled yellow curve); key emission regions are schematically represented in Fig. 1a. Source: reproduced from Fig. 1.2 of Harrison (2014).</figcaption></figure>

2023; Agazie et al., 2023; Reardon et al., 2023; Xu et al., 2023). The first direct identification of merging SMBHs is expected in the next decade with launch of LISA (Laser Interferometer Space Antenna; Amaro-Seoane et al., 2017).

Large field of view and all-sky monitoring observatories (e.g., the *Swift* high-energy observatory; Gehrels et al., 2004), allied to rapid response follow-up facilities, have greatly opened up the parameter space to identifying exceptional short-lived phenomena such as tidal disruption events (TDEs; e.g., Komossa, 2015; Gezari, 2021), which are thought to be due to the accretion of stars in the immediate vicinity of a SMBH. The unrelenting progress in instrumentation and facility development over the last decade with larger field of view instruments, greater sensitivity, higher spatial, spectral, and temporal resolution, and greater multi-plexing has also provided us with a deeper panchromatic view of AGN and richer astrophysical insight. The last decade has also witnessed a huge increase in computing power allowing for more sophisticated and detailed simulations with greater spatial, temporal, and mass resolution over larger volumes with refined physical prescriptions.

A major motivation for this review was our “What Drives the Growth of Black Holes: A Decade of Reflection” workshop held in Reykjavík, Iceland, over 26^th–30^th September 2022. This workshop was the culmination of a series of meetings organised over the last decade (see Appendix A for details). The sessions of the Iceland workshop were arranged to be identical to those of our original “What Drives the Growth of Black Holes?” meeting from 2010, which was the inspiration for the AH12 review, to facilitate a more direct assessment of the growth of scientific progress. Briefly, the four scientific sessions of the workshop addressed the following key questions:

* How does the gas accrete onto black holes, from kilo-

parsec to sub-parsec scales?

* What are the links between black-hole growth and their host galaxies and large-scale environments?

* What fuels the rapid growth of the most massive (and also the first) black holes?

* What is the detailed nature of AGN feedback and its effects on black-hole fuelling and star formation?

In this article we first describe the approach we take in this review, including a brief primer on how we can measure scientific progress (§2). In the subsequent four sections (§3–§6) we then address each of the key questions above, providing an overview of the research field and highlighting the significant areas of progress over the last decade. In §7 we then briefly reflect on the progress made in each of these research fields, utilising citation network analyses to quantify the relationships between different sub fields, and comment on expected progress over the next decade. In §8 we provide a brief summary and conclusions. As with all review articles, we are unable to assess every relevant paper in the scientific literature. Our aim is more to provide a clear overview of this broad research field and to provide a starting point for more detailed investigations.

## 2. Approach taken in this review

In this section we first provide an overview of the broad black-hole (BH) research field (§2.1) and show where the AH12 review (and, by association, this review) sits within that broader research framework. We then ask the question what exactly *is* scientific progress (§2.2), and define practical categories that we can use to track and identify scientific progress along with examples of each category (§2.3; see Table 1).

### 2.1. *Scientific context: the broader black-hole research field*

To place this review and the AH12 review into context, we first consider the much broader field of BH research, covering not only AGN and growing SMBHs but also more fundamental theoretical BH research. To demonstrate this, we use an approach called citation network analysis, which applies graph theory and network science to bibliographic data to identify clusters of papers that are densely connected through citation links. As has been shown in prior research (Klavans and Boyack, 2017; Leng and Leng, 2021), these clusters correspond to specific sub-fields and topics, reflecting the tendency for papers on a particular topic to cite other works in the same research area more frequently than those addressing different questions.

To construct the citation network covering black-hole research, we selected all peer-reviewed articles in indexed astrophysics journals from the Web of Science that contained terms indicating a focus on BHs or AGN/quasars, and which were published between 2003 and the first quarter of 2024. After parsing out the bibliographies of these papers, we constructed a network containing 60,924 papers connected together
<figure><img src="image_4.png" /><figcaption>Figure 3: Citation network diagram (CND) for (a) papers related to black-hole research with connections between papers plotted in grey. The location of AH12 across the CND is indicated by a red circle and the location of papers that either cited or referenced by AH12 are plotted using blue circles. (b) the 13 identified clusters across the CND plotted in different colours. The most closely associated cluster to AH12 is “AGN, quasars, and galaxies”, although it also has good association to the “Seyfert galaxies, emission lines”, and “Radio galaxies, clusters” clusters indicated in red, blue, and cyan, respectively. The location of three influential studies across the CND are also plotted as white circles, along with AH12: Kormendy and Ho (2013); Event Horizon Telescope Collaboration et al. (2019b, 2022).</figcaption></figure>

by 1,378,057 citation links. We then applied modularity maximisation to this network using the Leiden algorithm to detect clusters of densely interconnected papers (Traag et al., 2019): overall 13 clusters are identified. See Appendix B for more details about the construction of the citation network.

Fig. 3 shows the full citation network for BH research, where nodes represent papers coloured by membership of particular clusters that relate to specific topics and sub fields. The relative locations of these nodes to one another is based on their citation relationships - nodes that are interconnected by a citation are pulled together, with those without a citation link pushed apart (Jacomy et al., 2014).<fnref n="2" /> As can be seen from the top panel, the AH12 review is towards one side of this citation network and tightly associated with the second largest sub-field cluster which is related to research focused on “AGN, quasars, and galaxies”; however, it also connects more sparsely to many other clusters. For context, the sub field on the opposite side of the citation network to AH12, the largest cluster, focuses on the theoretical fundamental properties of BHs including general

relativity, quantum gravity, and Hawking radiation. Interestingly, the cluster at the centre of the citation network is research related to studying the SMBH at the centre of our Galaxy, a relatively small and diffuse sub field whose results inform both fundamental BH research and also the broader AGN/quasar research community. The location of three influential articles focused on measuring the properties of SMBHs are indicated on the bottom panel of Fig. 3, showing their relative “domains” in comparison to AH12: Kormendy and Ho (2013); Event Horizon Telescope Collaboration et al. (2019b, 2022). More details about each cluster, including their sizes and associations to other clusters are given in Tables B.5 and B.6.

## 2.2. What is scientific progress?

Before considering our pragmatic approach to measuring scientific progress in this review we first consider the more fundamental questions: what is scientific progress, how can we recognize when it has occurred, how (if at all) can we quantify it, and what are the drivers of scientific progress?

In the philosophy of science, scientific progress has been characterised in a range of different ways. These needn't be thought of as competitor characterisations (Taylor and Vickers, 2017) since when science progresses, it can so in different ways at different times. Two prominent dimensions of scientific progress concern *epistemic* progress (increase in knowledge), and *functional* progress (e.g., solving scientific problems, or development of better conceptual frameworks; Bird, 2007). Whilst these two aspects of progress often go hand in hand, they can also come apart: advances in scientific knowledge needn't be functional advances (e.g., knowledge of a new exoplanet), and functional advances needn't be advances in scientific knowledge (e.g., the introduction of Dirac's delta function; Dirac, 1927).

When we are considering an *increase* in something (e.g., knowledge, or problem solving), the question naturally arises to what extent can we identify and measure that increase? We can describe new discoveries by concrete tokens; e.g., the first black-hole merger detected through gravitational waves (LIGO Scientific Collaboration et al., 2015; Abbott et al., 2016), or the much more gradual discovery that SMBHs play a significant role in the formation and evolution of galaxies, first postulated ≈ 20–30 years ago (e.g., Silk and Rees, 1998; Fabian, 1999; Bower et al., 2006; Croton et al., 2006). We could in principle list all such discoveries and add them up in an effort to determine how much knowledge we have gained in the last ten years. But there are two problems with using such a method for measuring progress.

First, there is a description problem: there are many ways of articulating the new knowledge that has come out of, e.g., a single experiment, so that under one description we have “progress = 1” because we have only a single concrete token of new knowledge, but for another description we could have “progress = 50” if we broke down the information in that concrete token and separated it into fifty, smaller pieces of knowledge. This is especially obvious with discoveries that are more gradual, taking many years, since typically there are numerous mini discoveries that contribute to the broader discovery.

<footnote n="2">²A node refers to individual connections for a given paper while a cluster refers to all research identified to be associated to a specific sub field using citation network analyses.</footnote>
1. Facilities: technological advances due to new facilities; e.g., instruments, telescopes, observatories, super computers.

2. Data: increase in the amount of available data from existing facilities; e.g., larger (or more complete) data samples.

3. Techniques: new data analysis approaches leading to new insight; e.g., machine learning, Bayesian methods.

4. Community: group sociology, interactions, sizes, facilitated workshops and discussions, and potential for impact and funding.

5. Conceptual: new ideas from existing data; e.g., cross disciplinary research, inspiration, focused discussions (e.g., at workshops).

<caption>Table 1: Short description of the categories used in this article to indicate the drivers of scientific progress. The category labels are highlighted in bold.</caption>

Second, and perhaps more importantly, not all progress is equal. Some progress can be characterised as incremental while other areas of progress can be considered “revolutionary”, which itself could be sub-categorised into “major” and “minor” varieties. We might also think that one conceptual revolution is somehow *more progress* than a thousand incremental pieces of new knowledge. The same could be said of progress understood as problem solving, or conceptual developments. Such distinctions and subtleties are important, since the same piece of research could be packaged in radically different ways vis-a-vis its contribution to scientific progress, and biases could influence the choice of packaging.

Viewed from a distant standpoint, and learning from the history of science, some cases of apparent scientific progress actually turn out not to be progress at all, or at least, only in a thin sense. For example, the phlogiston paradigm of combustion, the caloric paradigm of heat, pre-evolutionary biology, the ‘fixist’ paradigm prior to acceptance of continental drift, and the steady state cosmological model prior to the detection of the cosmic microwave background (CMB): these are all instances of apparent scientific progress that is now largely forgotten, considered irrelevant or misguided ever since a subsequent scientific revolution. Thus, the question arises to what extent, if at all, can we distinguish apparent progress from genuine progress? We may also effectively think that the major questions in some fields are addressed and all that remains is to tidy up the details, but history has taught us that revolutions in apparently mature fields can happen (Kuhn, 2012).

However, the idea that all scientific progress should only be considered ‘apparent’, on the grounds that we cannot rule out a future scientific revolution that completely reshapes the field, is quite extreme. Arguably, there are many examples of ‘solid’ scientific knowledge, where major revolutions are not only unexpected, but rather implausible (Godfrey-Smith, 2003, p. 98); Vickers (2022). Even in BH research - one of the purest examples of “remote science” - much is now known with a high degree of certainty. For example, today it is an established scientific fact, supported by overwhelming evidence, that the energy from an AGN is produced by material being accreted onto a SMBH but it took many decades to firmly establish this fact.

## 2.3. Scientific progress: approach adopted in this review

After we accept that we can identify instances of scientific progress, the question remains: how can we practically measure or quantify the amount of progress, both across the broad research field, and also within specific topics and sub fields?

In the absence of any widely accepted approach to quantify or otherwise measure scientific progress, in this review we adopt a “know it when you see it” approach. That is to say, we rely on specialists working in the field, as well-placed to identify both areas where there has been significant progress and areas where there has been a notable lack of progress, using the AH12 review as a snapshot of the field back in 2012 to provide a convenient baseline from a decade ago. In each of the subsequent four sections (§3–6) we attempt to highlight where significant progress has, and has not, been made over the last decade. In cases where we identify significant progress, we also consider the likely drivers of this progress; in this way, we can also develop hypotheses concerning how these drivers might potentially be manipulated, in order to optimise the possibility of progress or to increase the efficiency of progress in the context of future research (see §7.3).

While the drivers of scientific progress are neither discrete nor completely enumerable, we have postulated five main types of drivers of progress within scientific research (see Table 1). Some of these drivers are easier to recognise than others, and should lead to demonstrable scientific progress: for instance new facilities (category 1), the compilation of larger and more comprehensive datasets (category 2), and the development of new data-analysis techniques (category 3). Additionally, we suggest that developments in community engagement (category 4) and conceptual shifts (category 5) can be important drivers of progress, but often in more subtle ways than categories 1-3. They can be harder to quantify precisely because they are the places where amorphous phenomena such as “serendipity” occur: you cannot predict when two people having a conversation over coffee will stumble upon a novel concept. However, one proxy for community engagement is citation networks, which we introduced in §2.1, and will utilise again in §7.2.

To flesh out these progress-driver categories (Table 1), we list some examples below.

* **Category 1 (facilities):** this category relates to significant progress enabled by new facilities that have greatly opened up new regions of parameter space through orders of magnitude greater resolution (spatial, spectral, temporal), spatial coverage, and sensitivity. A clear example of this is JWST (Gardner et al., 2023) which within a couple of years of operations is already driving huge advances in our understanding of AGN activity across all redshifts, particularly at high redshift, due to unprecedented sensitivity and unattenuated coverage at near-IR–mid-IR wavelengths.

* **Category 2 (data):** this category relates to significant
progress enabled by significant increases in data from ex-
isting facilities. An example of this category is the increase
in the number of quasars spectroscopically identified from
the Sloan Digital Sky Survey (SDSS): ≈ 106 k around
the time of the AH12 review (Schneider et al., 2010) to
≈ 750 k from the latest quasar data release (Lyke et al.,
2020) and the ≈ 3 M quasars expected from the DESI sur-
vey (Chaussidon et al., 2023).

*   **Category 3 (techniques):** this category relates to significant progress enabled by developments in data-analysis approaches. This category is quite often closely related to category 2 since a huge increase in data volume requires the adoption of techniques to efficiently analyse such large data sets. A good example is machine learning and neural network based techniques: over an order of magnitude more published astronomy papers have adopted these techniques in the last decade.<fnref n="3" />

*   **Category 4 (community):** this category relates to developments facilitated by community engagement, which can be powerful drivers of progress. This is especially obvious when research groups, previously largely isolated from each other (e.g., working in different fields or disciplines: inter disciplinary), start to interact at regular focused meetings, with novel research materialising as a direct result. An example is the Lorentz Center workshop in Leiden 16–20 October 2017, which led to a series of influential articles explaining the myths and reality of AGN feedback (e.g., Cicone et al., 2018; Cresci and Maiolino, 2018; Harrison et al., 2018; Wylezalek and Morganti, 2018).

*   **Category 5 (conceptual):** this category is more nuanced. Conceptual progress is often easy to identify; for example, seeing AGNs as typical “events” in the lifetime of galaxies, rather than as rare or unusual phenomena, represents progress in the form of a conceptual shift. But there is likely to be another key *driver* of this progress relating to any of the other categories; e.g., new facilities, more data, improved analysis techniques, facilitated community engagement possibly through inter-disciplinary research. However, the resultant new conceptual framework can subsequently be defined as a driver of progress.

3. How does matter accrete onto black holes from kilo- parsec to sub-parsec scales?

AGN activity requires a supply of gas. In this first scientific
section we consider how the gas is driven from the kilo-parsec
scale of the host galaxy to the SMBH radius of influence and
onto the accretion disk to be accreted by the SMBH.

The fuelling of the SMBH is confronted with a huge angular
momentum problem, regardless of the abundance of the avail-
able gas reservoir. All scales are concerned by this angular mo-
mentum problem, from the outer galaxy disk (10 kpc scale), to

the sphere of influence of the SMBH (pc scale), down to the accretion disk scale (0.01 pc); see Fig. 1. Low-mass black holes may accrete stars in their close vicinity, especially those with low angular momentum; i.e. in almost radial orbits. Stars that have a low angular momentum, and can interact strongly with the black hole, come from a region in phase space known as the loss-cone (Frank and Rees, 1976), since they will be swallowed by the black hole. When these stars are depleted, the relaxation time to refill the loss-cone is long. The relaxation time is the time-scale necessary for a star to lose its energy through encounters with others. In a galaxy disk, the relaxation time is much larger than the Hubble time, although it could be shorter in dense regions, like globular clusters, or nuclear regions. To fuel the black hole, it is more efficient to rely on the gas component, which can cool and dissipate energy. It may lose angular momentum through gravity torques from the non-axisymmetric potential, due to stellar bars and spirals.

The angular momentum of matter in rotating galaxy disks is proportional, per unit mass, to the circular velocity $V_c$ and to the radius $R$. A typical rotation curve in a spiral galaxy can be decomposed into roughly three regions: a central SMBH-dominated zone ($R_{\text{Rol}$; see Footnote 1), where the potential is Keplerian with $V_c \propto R^{-0.5}$, a rising velocity zone dominated by the bulge $V_c \propto R$, and finally a flat rotation curve zone $V_c=V_{\text{flat}}$, due to the conspiracy of the stellar disk and dark matter halo potentials. The corresponding specific angular momentum ($J = R \times V$) always increases with radius with a power-law dependence with slopes of 0.5, 2 and 1 for these 3 zones, respectively; see Fig. 4.

For the material within the central kpc region of the galaxy, it will need to lose a factor of at least 1,000 in specific angular momentum to reach the accretion disk at a radius of ≈ 10⁻² pc. The amount of gas inflow (dM/dt), to account for the AGN luminosity, assuming a radiative efficiency of 10% and a luminosity L = 0.1 dM/dt c², would need to vary from 10⁻<fnref n="3" /> M☉ yr⁻¹ to 2 M☉ yr⁻¹ for the most luminous quasars of 10¹<fnref n="3" /> L☉. Assuming a typical duty cycle of the order of 100 Myrs, the SMBH would therefore have to accrete about 2 × 10⁸ M☉ during its “active” period, and the gas has to inflow within just a few dynamical time-scales, since the rotation period at 1 kpc radius is ≈ 30 Myr. Very efficient dynamic mechanisms are required to exchange the angular momentum at those scales in the galaxy. Normal gas viscosity is negligible in this region, given the low density of clouds (e.g. Lin and Pringle, 1987). If viscous torques are negligible, only gravity torques can drive the gas inwards, and make it lose angular momentum. These torques are due to the tangential forces, produced by non-axisymmetric disturbances, such as bars. Stellar bars create a robust and long-lived bi-symmetric potential, that is able to drive the gas inwards. The stars will then gain the angular momentum, and the bars will weaken (Bournaud and Combes, 2002). From optical images, two thirds of spiral galaxies are classified as barred, and from near-infrared images, more nuclear bars are detected, that were not seen before (e.g. Salo et al., 2015). Given that bars can disappear and form again, it is also likely that bars have developed in every galaxy at some time during their evolution.

To better understand how the gas can be driven inwards to

<footnote n="3">³On the basis of a search for the word “machine learning” in the abstract field using NASA ADS: ≈ 1,400 refereed papers in 2012 and ≈ 18,400 refereed papers in 2023.</footnote>
<figure><img src="image_5.png" /></figure>

Figure 4: Rotational velocity and angular momentum versus radius around a SMBH. Top panel: the SMBH dominates the inner rotation curve (Keplerian potential), which is decreasing; then the curve rises due to the bulge, and eventually reaches a plateau due to the conspiracy of the gravitational potential of the disk and the dark matter halo (DM). The different colors mark these various contributions; Bottom panel: the corresponding angular momentum per unit mass J, increases as power-laws of the radius, with the indicated slopes: 1/2 (Keplerian), 2, and 1 for the three zones.

<figure><img src="image_6.png" /></figure>

Figure 5: Left: Main stellar orbits in the bar rotating frame. The bar is along the horizontal axis. Note the x1 orbits, parallel to the bar, until $x = 6$ kpc. At the corotation radius, the orbits are restricted to epicycles around the Lagrangian points. Bar torques are positive in the red quadrants, and negative in the blue ones, with respect to the galactic rotation, indicated by the arrow in the top right corner. Right: Gas-cloud orbits, and spiral formation, triggered by the bar. The cloud-cloud collisions tilt the orbits, that turn gradually by 90° at each resonance. The elongated orbits of the left panel, with their respective colour, are now precessed to form a logarithmic spiral.

fuel the central SMBH, we describe the main dynamical features of a barred spiral galaxy, the orbital structure, the resonances, and the gravity torques between stars and gas in §3.1. Some fraction of SMBH growth can also be due to the accretion of stars, which is detailed in §3.2. A large part of the progress in the last decade in this research field has been facilitated by more sensitive observations and more extensive data, essentially through increased spatial resolution, brought by new facilities and interferometers. This will be described in §3.3, with ALMA (Atacama Large Millimeter Array) discoveries of molecular tori, kinematically and morphologically decoupled from the main galaxy disk, and with VLTI (Very Large Telescope Interferometer) mid-IR interferometric images, where the main dust emission near the AGN traces out a polar cone geometry. These observations are progressively changing our concepts and paradigm about the dusty torus and the AGN unification model. At even higher spatial resolution (sub-milliarcsec scales probing ~0.05 pc), the radio very long baseline interferometry, and the EHT, are now revealing several features of the accretion disk. Our knowledge of the SMBH growth mechanism on these scales is reviewed in §3.4. The small sizes of the AGN emission regions imply that they can vary on day-year time scales. The stronger variations may result from a complete transformation from an optical type 1 AGN (with prominent broad emission lines) to a type 2 AGN (lacking broad emission lines), and these changing-look AGN, extreme variability events, and quasar periodic eruptions teach us a lot about the SMBH accretion mechanisms, as described in §3.5 and §3.6. Numerical simulations and new techniques using ever more powerful computers have enlightened these mechanisms in the last decade, as shown in §3.7. Finally, §3.8 summarizes the progress made over the last decade, noting that the future is bright both in the high-spatial resolution and the time domain.

### 3.1. Gas infall due to bars and spirals

From a theoretical stand point, most of our understanding of gas inflow through bars and spirals was developed in the
<figure><img src="image_7.png" /><figcaption>Figure 6: Left: in the absence of a central SMBH, a leading spiral is expected inside the ILR. Right: in the radius of gravitational influence of the SMBH, all frequencies, and in particular the $\Omega - \kappa/2$ precessing frequency, are increasing with decreasing radius and the spiral inside ILR becomes trailing.</figcaption></figure>

decades prior to AH12, with the main progress over the last decade coming from observations, which confirmed some aspects of the theoretical predictions that were not previously testable. The non-axisymmetric disk potentials due to bars and spirals produce gravity torques able to drive the gas from their corotation inwards. The actual SMBH fuelling requires several steps, depending on the spatial scale. A primary bar of typical radius 5 kpc, can drive in a first step the gas from its corotation near 5 kpc to the inner Lindblad resonance (ILR) at 100 pc scale, where the gas accumulates in a ring. Being then aligned with the bar, the gas does not suffer any torque, and the gas inflow may be stalled. In a second step, a secondary bar may decouple from the primary bar, and continue its influence from 100 pc to 10 pc scales. The various non-axisymmetric features are characterized by their Fourier components: the surface density, or the implied potential, is decomposed in a series $A(r) = A_0(r) + \sum A_m(r) \cos[m(\theta - \theta_m(r))],$ where typically $m$ is considered up to the 6th harmonic. Bars have $m=2$ and a constant $\theta_m(r)$, while the latter gives the winding shape for a spiral. There can exist several embedded structures, mostly of $m=2$ morphology; however, close to the nucleus, when the SMBH Keplerian gravitational potential begins to dominate, $m=1$ (lopsided) features may contribute.

The sign of the torques can be derived from the main characteristics of stellar orbits in a bar potential (see Fig. 5, left). The stellar orbits are structured around a skeleton of periodic orbits, playing the role of building blocks, trapping orbits around them. The most important periodic orbits are the x1 family, parallel to the bar inside corotation, and the x2 family, perpendicular to the bar in between the two ILR, if they exist (see the review by Contopoulos and Grosbol, 1989). The existence of two ILRs within the epicyclic approximation might not be sufficient for the x2 family to appear. When the bar is strong enough, the x2 orbits disappear. Then the elongations of the orbits change by 90° at each resonance, so that outside corotation the orbits are perpendicular to the bar until the outer Lindblad resonance (OLR). They do not support the bar, which ends at about its corotation radius.

While stars are a collisionless component, and perpendicular orbits can coexist, the gas has a different behavior, because of dissipation. Gas clouds following periodic orbits collide with

each other, which tilts their orbits; they also change by 90° at each resonance, but gradually (see Fig. 5, right). The crowding of their stream lines produces a spiral morphology (Sanders and Huntley, 1976). The spiral is relatively open, and can wind at maximum by 180-360°, according to the presence or not of 2 Inner Lindblad Resonances (ILR), corotation (CR) and Outer Lindblad Resonance (OLR).

Since gas and stars are not aligned, stellar bars exert a torque on the gas, except at resonances, where the gas configuration falls back into symmetry with the bar. The bar splits the plane into four quadrants, where the gravity torques change sign, with respect to the sense of rotation of the galaxy (see Fig. 5, left). Between corotation and ILR, the gas spiral arms gather in the quadrants of negative torques, and gas is driven inwards, increasing the depth of the potential well. Then all frequencies, including $\Omega$ and $\Omega - \kappa/2$, the orbit precessing rates, increase towards the center and trigger the existence of two ILRs. The vertical resonance with the bar develops a peanut-shape bulge, weakening the bar further. This triggers the decoupling of a secondary bar, an embedded nuclear bar, rotating faster than the primary bar (e.g. Buta and Combes, 1996).

When the dissipative gas is driven inwards, the precessing rate of its elongated orbits $\Omega - \kappa/2$ first increases, while the radius decreases. This leads to a trailing spiral structure (see Fig. 6). In between the two ILR, the precessing rate then decreases, after having reached a maximum, and a leading spiral is expected. However, when entering the $R_{\text{Rol}}$ of the SMBH, all frequencies will increase again, since the potential becomes Keplerian, see Fig. 4. In this region, a trailing spiral is then expected, as shown in the right panel of Fig. 6. The torque, which was positive on the leading arm, and maintained the gas in the ring, then becomes negative again, providing the possibility to fuel the AGN.

## 3.2. Black hole growth through the accretion of stars

A small amount of SMBH fuelling, especially for gas-poor early-type galaxies, can come from stars in the vicinity of the nucleus. The capture and accretion of nearby stars will give rise to a flaring event, called a TDE, for Tidal Disruption Event (Kormossa, 2015; van Velzen et al., 2020; Gezari, 2021). Due to the rapid progress of time-domain astronomy over the last decade, transient event monitoring facilities like the Zwicky Transient Facility (ZTF; Kochanek et al., 2017), or the All-Sky Automated Survey for Supernovae (ASAS-SN; Bellm et al., 2019), have allowed for many TDEs to have been detected, which bring new and precious knowledge of the AGN accretion disks. In the near future the Vera Rubin Telescope will yield a wealth of such events (Hambleton et al., 2023), likely leading to significant growth in the size of the TDE community (see §7.2 for the growth of this community over the last decade).

When a star ($r_*, M_*$) with low angular momentum reaches the tidal radius of a central SMBH $R_T = r_*(M_{BH}/M_*)^{1/3}$, the star can be disrupted by tidal forces, and a large fraction of its gas becomes captured by the accretion disk. Assuming for the average stellar density, $\rho_* = 1.4 \text{ g.cm}^{-3}$ the average density of solar mass stars,
$$R_T(cm) = 6 \times 10^{13} M_8^{1/3} \rho_*^{-1/3}, \quad (1)$$

where $M_8$ is the black hole mass in units of $10^8 M_\odot$. As the SMBH event-horizon radius $R_S = 2 G M_{BH}/c^2 = 3 \times 10^{13}$ cm $M_8$, grows faster with $M_{BH}$ than the tidal radius, there is a limit, when $M_8 \sim 3$, above which the star disruption occurs inside the black hole, and there is no gaseous release or AGN activity (but the SMBH grows even more rapidly). This is the critical Hills limit (Hills, 1975). For SMBH masses lower than this limit, these capture events can be observed as X-ray flares, and/or optical-UV flares, during typically one year, their signature being a characteristic light curve decreasing with time with a -5/3 power-law slope (Gezari et al., 2012). These TDEs are expected to occur at the frequency of one every 10,000 years in the Milky-Way galaxy. Some TDEs have been detected in external galaxies, where their temperature appears lower than expected, and decreasing less rapidly in time. This could be due to feedback effects in the form of a disk wind, when the accretion rate is of the order of the Eddington limit (Miller, 2015). But soon, the depletion and loss cone effect dries up this fuelling source, unless galaxy interactions reshuffle the stellar distribution, and creates nuclear star clusters, in a nuclear starburst.

Several dozens of TDE have now been observed, and discovered not only in the X-ray, but also in the UV-optical (van Velzen et al., 2020). TDEs observed in both wavebands are rare. It has been advocated that the optical-UV emission is due to the reprocessing of the X-ray photons by material around the accretion disk. It could even be an orientation effect, if this material preferentially resides within the equatorial plane, as in the unified model (Dai et al., 2018a). Most TDEs occur in post-starburst or quiescent galaxies (French et al., 2020). They are not detected in AGN hosts, which may be due to the difficulty in distinguishing a TDE; i.e., the accretion of a single star would likely be lost within the emission produced by an AGN accreting a large amount of gas. The TDE might be confused with AGN flares, or there might be too much dust extinction to detect the TDE. Therefore, AGN hosts suffer selection biases against the detection of TDEs (French et al., 2020).

For the various TDEs already observed in detail, the luminosity decreases with time as a power law, as expected, although sometimes it depends on the determination of the time of the emission peak, which is not always observed. The peak intensity and the duration, from a few months to one year, reveals a large diversity. There is a good correlation between the fall-back time and the SMBH mass (estimated from the $M-\sigma$ relation), which means that the TDE observation can help to determine $M_{BH}$ (van Velzen et al., 2020). Optical spectra have helped to distinguish TDEs from AGN flares and from supernovae. Lines of hydrogen and helium are observed with different ratios varying with time, due to radiative transfer and opacity issues. The width of the emission lines vary with time, from initially $v \approx 10^4$ km s$^{-1}$ but becoming narrower on the timescale of a month. The broad width of the emission lines is interpreted as scattering by hot electrons, and the decrease with time as the decrease in the density and optical depth of the emitting region.

<figure><img src="image_8.png" /><figcaption>Figure 7: Top: Schematic representation of the main morphological features observed in NGC 613. The dust lanes are leading the large scale bar. The filaments are shown in dashed lines between the ILR star-forming nuclear ring and the nuclear trailing spiral. Bottom: the molecular torus (zoomed at left, in colour) is observed inside the nuclear trailing spiral traced by the black contours of ALMA CO(3-2) emission. Source: adapted from Figs 4, 5, & 16 of Audibert et al. (2019).</figcaption></figure>

The existence of outflows associated with TDEs is particularly revealed by broad absorption lines (BALs) in the UV spectrum, in particular in the ionized lines (C IV, Si IV), similar to what appears to be occurring in BAL quasars (see §5.2).

The TDE host galaxies have a higher central stellar concentration than other galaxies of the same morphological type, which is not unexpected (Hammerstein et al., 2021). However, the current small sample sizes do not exclude the possibility of observational biases. Observations until now allow for the estimation of the amount of mass accreted by the SMBH through TDEs, and this is at best a few percent of the total SMBH mass. Other sources of fuelling are therefore also required, in the form of gas accreted from the host galaxy, feeding normal AGN (Yuan and Narayan, 2014). Observations of TDEs are, however, key for probing otherwise dormant SMBHs, explore general relativistic effects in strong gravity, and the investigation of magneto-hydrodynamic gas streams near black holes. They are also an opportunity for reverberation mapping (van Velzen et al., 2021).

### 3.3. Fuelling the AGN: a review of recent observations

The key to progress in our understanding of the fuelling of SMBH growth is tracing the cold molecular gas and dust at high spatial resolution in the central region of galaxies. This
<figure><img src="image_9.png" /><figcaption>Figure 8: Left: Molecular gas distribution in NGC 613: inside the ILR ring, the CO(3-2) emission reveals a nuclear spiral. Middle: Corresponding velocity field, with a major axis oriented almost East-West. Right: zoom into the molecular torus, inside the nuclear trailing spiral. The HCO$^+$ (4-3) velocity field has a major axis oriented almost North-South. This HCO$^+$ torus has the same extent as the zoomed emission in colour of the left bottom panel of Fig. 7. The scales are in arcsec (1" = 83 pc), and km/s. Source: adapted from Figs 2 & 7 of Combes et al. (2019) and Audibert et al. (2019).</figcaption></figure>

has been made possible thanks to recent large interferometric
facilities, in both infrared (IR) and millimeter wavelengths; see
Fig. 2 for waveband definitions. At millimeter wavelengths, it
is possible to reach a resolution of ~13 mas with ALMA, and
much more, ~40 µas with the very long baseline of the EHT
(Event Horizon Telescope Collaboration et al., 2019a). In the
near-IR waveband, the maximum resolution is 3 mas, corre-
sponding to just ~0.22 pc, at a typical distance of D = 15 Mpc,
for nearby spiral galaxies, sufficient to map out the emission
from the hot dust in the vicinity of the AGN.

The advent of ALMA in 2011 has been instrumental in ad-
vancing our empirical understanding of gas inflows down to
small (< 10 pc) scales over the last decade (García-Burillo et al.,
2014, 2021). It is now possible to trace molecular gas not only
in the primary and nuclear bar regions, as expected by the dy-
namical theory presented in §3.1, but also inside the nuclear
spiral structures, which has allowed for the discovery of addi-
tional molecular disk components. The latter has been found
to be randomly oriented with respect to the rest of the disk,
and is kinematically decoupled: we associate this structure to
the molecular torus, which is central to the unified AGN model
(Urry and Padovani, 1995). The gas fuelling of these molecular
tori is thought to be the result of secular evolution, with the nu-
clear spiral being the last step in the fuelling process (cf §3.1).
The molecular tori is within the region where the SMBH domi-
nates the gravitational potential (i.e., at $R_{\text{Rol}}$, see Footnote 1 and
Fig. 1). The accretion time scale onto the molecular tori within
this region is quite short (< 10 Myr), and nuclear bars are more
ephemeral than primary bars. However the decoupling of an
embedded bar weakens the primary bar, and also the positive
torques, outside corotation. This allows the replenishment of
the gas disk, so that the molecular torus may be fuelled inter-
mittently.

Many barred Seyfert galaxies, like NGC 613, have been
found to be in a feeding phase, fuelling the molecular torus (cf.
Fig. 7), with a nuclear trailing spiral inside the ILR ring, as ex-
pected when the gas enters the radius of gravitational influence
of the SMBH, as in Fig. 6. The computation of the torques
has confirmed the evidence of fuelling the molecular torus in

NGC 1566 (Combes et al., 2014), NGC 613 (Audibert et al., 2019), and NGC 1808 (Audibert et al., 2021).

NGC 613 is one of the prototypical cases of a strong barred
galaxy, with a star forming gas ring at ILR. The first ALMA ob-
servations, with moderate spatial resolution, could see only the
ring, without resolving the internal structure (Miyamoto et al.,
2017). The computation of the torques indicated that the gas
can lose all of its angular momentum in only one rotation, when
inside a ≈ 50 pc radius. The observed molecular gas velocities
show that the molecular torus, at the center of the nuclear spiral,
has decoupled kinematics; i.e., the kinematic major axis is not
aligned with that of the large-scale disk, see Fig. 8.

Conversely, Fig. 9 shows an example of a nuclear spiral,
but without the molecular torus in NGC 1097; the latter could
have been suppressed by AGN feedback, see §6. Indeed, it has
been shown that the cold molecular gas concentration, at scales
below 50 pc, is quite high compared to the 200 pc-scale molec-
ular surface density but only below a critical X-ray luminosity
(García-Burillo et al., 2024). Above this critical luminosity, the
concentration falls by 4 orders of magnitude. This may be at-
tributed to AGN feedback, which depletes the central gas. This
result has been obtained for a sample of 64 nearby (D = 7–
45 Mpc) disk galaxies including 45 AGN and 19 non-AGN
(García-Burillo et al., 2024), where there is sufficient spatial
resolution to identify a molecular torus, when present. Addi-
tional data is being acquired to improve the source statistics.

The original AGN unification paradigm was based on the ex-
istence of a dusty torus whereby the broad line region (BLR)
and accretion disk of a Type 1 AGN was obscured if the dusty
torus lay along the line of sight towards the observer, such that
only the Type 2 AGN features are identified. However, high-
resolution infrared observations, including ground-based inter-
ferometers (in particular VLTI-MATISSE) have shown that the
dust in the central parts of AGNs can often be detected in the
polar direction, instead of being aligned in the same plane as the
putative dusty torus (Asmus et al., 2016; Asmus, 2019; Left-
ley et al., 2021, 2024). In a revised AGN unification scenario,
a conceptual advance driven by new facilities, the obscuration
can come both from a dense and geometrically thin dusty disk
as well as the base of the polar cone, which is quite clumpy
(Isbell et al., 2022, 2023), with the geometrical thickness from
the combination of these obscuring regions corresponding to
the relative abundance of Type 1 and Type 2 AGN (Alonso-
Herrero et al., 2011; Ramos Almeida et al., 2011; Elitzur, 2012;
Ramos Almeida and Ricci, 2017). The polar dust component
is most likely ejected (or even formed) within an AGN-driven
wind (Alonso-Herrero et al., 2021), starting from the dust sub-
limation radius (pc size), and forming the border of a hollow
cone (Hönig, 2019). The gas is therefore initially predomi-
nantly in infall within a geometrically thin disk identified as
the molecular torus, but can later be in outflowing in a direction
perpendicular to the torus and the accretion disk. However, po-
lar dust is not found in all AGN (for example, observations of
two high-Eddington ratio sources show no evidence for polar
dust; Drewes et al., 2025) and it is expected to be most com-
monly identified in sources with accretion properties that are
conducive to a long-lived extended outflow (e.g., Venanzi et al.,
<figure><img src="image_10.png" /></figure>

Figure 9: Zoom-in on the nuclear spiral observed with ALMA inside the ILR ring of NGC 1097. The left image is a large-scale UV image of the galaxy with *Galex*, the middle is an HST image of the star forming ring at ILR (Fathi et al., 2006), and at right is the CO(3-2) map from Izumi et al. (2017). *Source:* adapted from Fig. 3 of Izumi et al. (2017).

2020; García-Bernete et al., 2022). Molecular outflows are more difficult to detect along the colder molecular gas, and/or could be dragged by radio jets, which are usually oriented parallel to the wind direction, as sketched in Fig. 10.

Further key progress has been facilitated in this research field by Integral Field Units (IFU) like MUSE on the VLT, which can map the kinematics of the ionized gas across the galaxy (Husemann et al., 2019; García-Bernete et al., 2021; Juneau et al., 2022). These kinematical studies have been of paramount importance to distinguish between rotational motions of the gas and outflows, that could produce efficient feedback, as discussed in more detail in §6, Outflows accelerated by AGN are commonly observed in the form of coherent, mildly collimated high-velocity gas directed along the AGN ionisation cones, and kinetically powerful jets (García-Bernete et al., 2021). The outflow is comprised of gas in different phases (i.e., it is multi phase), with the highest velocity gas being ionized but where the bulk of the mass in the outflow is in the molecular phase (Fiore et al., 2017). If a jet component is present and lies perpendicular to the disk of the galaxy then its impact will be at a maximum, due to a stronger coupling with the ISM gas, even more so if the jet has a low power (i.e., non relativistic) since it can interact with the ISM for a longer duration. This scenario appears to be occurring in NGC 1068, the prototypical nearby Seyfert 2 (García-Burillo et al., 2016, 2019) where the jet-ISM interaction drives a cocoon in the galaxy disk, and produces lateral velocity perturbation perpendicular to the jet direction (Venturi et al., 2021; Girdhar et al., 2022). The gas excitation and broadening of the emission lines is predominantly due to shock ionization. An ionized collimated outflow with symmetric blueshift and redshift may exist in the direction parallel to the jet and wind. Complex situations may occur when the jet and wind are completely aligned with the galaxy disk, entraining ionized and molecular gas outflows, while an oblique ionization cone can also be identified where the ISM is elevated in a fountain by the star formation feedback and illuminated by the central AGN (Husemann et al., 2019). For each outflow “event”, the AGN feedback is not sufficiently energetic or efficient to quench star formation on the galaxy scale, and would have a more local impact. However, when repeated outflows occur, their cumulative effect may well be crucial to slowly quenching star formation; see §6.

High resolution near-IR AGN imaging of NGC 1068 has

<figure><img src="image_11.png" /></figure>

Figure 10: Schematic view of the circum-nuclear environment of the SMBH, with the radio jet, the molecular circum-nuclear disk, or molecular torus, the hollow dusty polar cone shaped by the AGN wind, and molecular outflows tracing the cone. In some nearby AGN, the circum-nuclear disk contains H₂O maser emission. The density of the disk is expected to decrease exponentially with radius and height. *Source:* adapted from Fig. 4 of Hönig (2019).

also been obtained with VLTI-GRAVITY (GRAVITY Collaboration et al., 2020). The interpretation of the results in terms of a circum-nuclear ring has been debated by Gámez Rosas et al. (2022). Finally, combining GRAVITY and MATISSE data has resulted in a coherent multi-wavelength model, where the parsec-scale region is composed of a dust disk plus a polar wind (Leftley et al., 2024). Breakthrough results were also obtained for M 87 using the Global Millimeter VLBI Array (GMVA) complemented with ALMA, and Greenland Telescope (GLT) observations to not only image the shadow of the SMBH (see §1), but also the radio jet at the base where it is formed and launched, showing that it produces edge-brightened structures connected to the accretion flow and the gravitationally lensed rings (Lu et al., 2023). In Centaurus A, it was also possible to reveal the launching and initial collimation region of the radio jet, down to 10 gravitational radii (Janssen et al., 2021). As in M 87, the radio jet is found to be edge brightened, and its structure is remarkably comparable, when scaled by SMBH mass, leading to the potential identification of a universal profile for SMBHs over a range of scales.

To further progress our understanding of the AGN feeding and growth of SMBH, there are at least two clear approaches to follow: first, to obtain high resolution data of ionized, molecular gas and dust on a larger sample of systems allowing to then calculate timescales and duty cycles, in comparison with simulations of the circum-nuclear regions, and in particular producing the torus; and second to obtain much higher resolution observations, namely with VLBI revealing water maser emission like in Fig 10, closer to accretion disks, where warping and differential precession dominate (Greenhill et al., 1997; Herrnstein et al., 1999; Gallimore et al., 2004, 2024).
### 3.4. The accretion disk

How is the gas accreted towards the SMBH at very small, sub-parsec scales? Theoretically we would expect the structure and geometry of the accretion disk to depend on the accretion rate, with respect to the Eddington rate. The latter corresponds to the Eddington luminosity, which is proportional to the mass of the black hole, i.e. $L_{Edd}/L_{\odot} = 3.2 \times 10^4 M_{BH}/M_{\odot}$. This is the maximum luminosity reached by an AGN, where radiation pressure from the accretion disk balances the gravitational force, assuming a spherical geometry. In a typical highly accreting AGN, where the accretion rate is just below the Eddington limit (typically expressed as the Eddington ratio $\lambda_{Edd} = L_{Edd}/L_{\odot}$ between 0.01 and 1), we expect accretion to occur in a classical geometrically thin rotating accretion disk, where viscous torques transport angular momentum away. The disk is optically thick, relatively cold (i.e., its temperature between $10^3$ and $10^4$K in the outer parts, is below the virial temperature, of order $10^9$K), and radiating efficiently, like a black-body. When the accretion rate is above the Eddington limit, then the disk is expected to become “puffy” (i.e., geometrically thick) and cannot radiate efficiently, and some of the energy is accreted into the SMBH by advection. This is often referred to as a slim disk (e.g. Kubota and Done, 2019); see Fig. 11. This accretion regime may explain in particular Narrow Line Seyfert 1s (NLS1s), which have broad emission lines that are narrower than typical Seyfert 1s (Wang and Netzer, 2003). Note that the different states of accretion flows have been established first for X-ray binaries, where they are more easy to establish due to much shorter timescales (e.g. Fender et al., 2004).

At the other extreme, if the accretion rate is low, $\lambda_{Edd} < 0.01$, then the disk is less optically thick and can even become optically thin, and again can no longer radiate efficiently, becoming a RIAF (Radiatively Inefficient Accretion Flow) (e.g. Yuan and Narayan, 2014). The energy instead heats the gas rather than being radiated, and towards the SMBH the medium becomes hot and geometrically thick. If an AGN transitions from a high accretion state to a low accretion rate then the thin disk is expected to be destroyed, initially near the SMBH and then moving out towards the edge of the accretion disk. In this low accretion rate state, energy can also be advected directly into the SMBH, and this regime is called an Advection Dominated Accretion Flow (ADAF) (e.g. Lasota, 2016). The radial velocity of the accretion is higher, since it will be proportional to the scale height of the medium, and also the rotational velocity will be less due to the gas pressure. This regime is included within the broader concept of “hot accretion flows”, which also host gaseous outflows and convection (e.g. Yuan and Narayan, 2014). The hot medium or hot “corona”, of size $\approx 3–10$ gravitational radii that forms as a by-product of the accretion process (Fabian et al., 2015), is responsible for the emission of hard X-rays through inverse Compton emission, as shown in Fig. 11. The hard X-ray spectrum is characteristic of a low-luminosity regime. This is shown in Kang et al. (2024) and Hagen et al. (2024), with observational evidence of changes in accretion disk SED with Eddington ratio, in particular see Fig. 5 of Hagen et al. (2024).

<figure><img src="image_12.png" /><figcaption>Figure 11: Accretion flows can have different configurations, according to the Eddington-normalized accretion rate, indicated in the right vertical line. At the bottom, with low accretion rate, the density of the flow is so low that its cooling is not efficient: the flow heats and becomes a geometrically thick cloud. At higher accretion rate, a cold and geometrically thin disk begins to form in the outer parts, while the center is an ADAF, with a radio jet. At quite high accretion rate, the cold thin disk extends down to the last stable orbit, and is efficiently radiating. Above this accretion rate, the luminosity is so high that the radiation pressure triggers a disk wind. Source: adapted from Esin et al. (1997) and redrawn and presented in Fig. 4.4 of Müller (2004).</figcaption></figure>

The physics of AGN feedback is expected to be very different across these accretion regimes (cf §6). At high Eddington ratio, radiation pressure on the ionized gas, close to the SMBH will drive very high velocity winds, observable as broad absorption lines in X-rays (Tombesi et al., 2010) and in the UV/optical (Borguet et al., 2013). At lower-luminosity ($\lambda_{Edd} < 0.01$), lower-velocity winds are expected to be launched from the accretion disk at larger radii by radiation pressure on dust (Ricci et al., 2017c). Kinetic feedback can also be produced via collimated outflows commonly observed as magnetically driven radio jets, which drag out atomic and molecular gas at lower velocities but which contains most of the mass of the outflow. This may happen for any Eddington ratio, provided that a radio jet exists. Radio jets are most frequently seen in early-type galaxies, and are related to the SMBH spin (Chiaberge and Marconi, 2011; Kormendy and Ho, 2013). Several regimes can be identified, corresponding to the large diversity of optical and UV observations, especially when considering line driven accretion disks winds, when most of the opacity is occurring in the spectral lines (Giustini and Proga, 2019). This can explain inactive nuclei, low-luminosity AGN (the most frequent in nearby galaxies), Seyfert galaxies, NLS1s and up to the broad absorption line quasars (BALQSOs); see also §5.2.

Another important phenomenon to take into account is the spin of the SMBH. Measurements through X-ray lines and reflection spectroscopy have shown that the spin is frequently quite high, and close to the maximum in AGN (Reynolds, 2013). In the future, the identification of photon rings or the last stable gas circular orbit with high resolution (EHT or VLTI-GRAVITY, Event Horizon Telescope Collaboration et al.
(2019b); GRAVITY Collaboration et al. (2018)) also have the potential to directly measure spin. This is important to confirm, since one of the main mechanisms to launch a radio jet is a rapidly spinning SMBH (Blandford and Znajek, 1977). The gravity of a rotating black hole exerts a relativistic “frame-dragging” effect on the surrounding gas, causing a warp and Lense-Thirring precession. The gas infalling from the galaxy disk is likely to have a random orientation with respect to the black hole spin, and will be submitted to the torque of the SMBH, which will have the effect of aligning it within a disk perpendicular to its spin (Bardeen and Petterson, 1975). The inner and outer disk are no longer aligned, and the disk develops a warp out to ≈ 100 $R_s$ (see Footnote 1). This warping is a characteristic feature of the water masers detected in nearby AGN accretion disks (Kartje et al., 1999; Gallimore et al., 2004; Caproni et al., 2006). The high spatial resolution brought by VLBI has revealed polarised emission of the water masers in the NGC 1068 accretion disk (Gallimore et al., 2024). The derived orientation of the magnetic field with respect to the accretion disk suggests that the large-scale outflow may be triggered in the accretion disk, through hydromagnetic instabilities.

If the viscosity of the disk is insufficient, instead of warping it may split into several rings, with different inclinations and precessing rates. When the various rings, with differential precession, collide, they will more rapidly drive the gas inwards toward the SMBH (Nealon et al., 2015; Pounds et al., 2018). General relativistic magnetohydrodynamic simulations of very thin disks around rotating black holes reveal the tearing of the disk. The torques from the frame dragging wins over the disk viscosity, due to magnetized turbulence, fragments the disk into a rapidly precessing inner part and slowly precessing outer part (Liska et al., 2021). The inner disk aligns perpendicular to the SMBH spin, while the outer disk keeps the “souvenir” of its unaligned angular momentum.

### 3.5. Transient phenomena: changing look AGN

AGN have been known for a long time to be variable over timescales of days to months and years, in particular in the optical/UV and X-ray wavebands, where the emission regions around the SMBH are small. X-ray spectra show absorption and emission features, mainly from reflection, which helps to derive sizes (Turner and Miller, 2009). However, in the last decade, more variability has been discovered, with greater amplitude, and strongly variable objects are called changing-look AGN (e.g., Ricci and Trakhtenbrot, 2023). These objects show dramatic flux and spectral changes, in both X-ray and optical/ultraviolet bands, changing optical class from Type 1 to Type 2 (or vice versa). These objects are challenging the unification AGN model; their number has been growing rapidly over the past decade, from initially a few examples to now a few hundred (Ricci and Trakhtenbrot, 2023; Guo et al., 2024b), With the advent of new facilities and surveys focused on the transient universe, like the ZTF, ASAS-SN and multi-epoch wide-area X-ray surveys like e-ROSITA (Merloni et al., 2020) combined with extensive optical spectroscopy of AGN and quasars over at least two epochs. In the future, the time domain will be further developed with new instruments, in particular the Vera

Rubin telescope (Hambleton et al., 2023), the *Einstein Probe* (Yuan et al., 2015), *ULTRASAT* (Ben-Ami et al., 2022), and these variations will bring new insights in the physics of active nuclei, and in particular on the unification scenario for AGN.

There exist two main mechanisms to explain the changing-look phenomena: one is changing obscuration, where the line-of-sight column density of gas and dust varies on short time scales due to the clumpiness of the obscuring medium, and the other is an intrinsic changing accretion state, due to variable accretion. In the first scenario, the obscuring clouds are expected to have a large velocity and could be associated with outflows transiting over the core of the AGN (e.g. King and Pounds, 2015), as well as the outer regions of the torus. Sensitive X-ray monitoring campaigns have revealed that changes in the absorbing column density towards individual AGN are common over timescales of days to years (e.g., Risaliti et al., 2002; Markowitz et al., 2014; Pizzetti et al., 2025). However, significant absorption changes corresponding to a unobscured-obscured or obscured-unobscured transition are far less common (Ricci and Trakhtenbrot, 2023); a significant changing-obscuration event in the Seyfert 1 galaxy ESO 323-G77 by Miniutti et al. (2014) is shown in Fig. 12. In the second scenario, where no obscuration is detected in X-ray, or when the shape of the UV-optical continuum slope implies no dust obscuration, the changing look may be explained by a changing accretion rate. Both the continuum emission and broad emission lines may appear and disappear, and the AGN may change from a type 1 to a type 2 and reverse back on the timescale of years-decades. This implies a significant change in the accretion rate.

One prototypical example of a changing look AGN is Mrk 1018. When discovered, Mrk 1018 was classified as a Type 1.9 Seyfert galaxy, although it transitioned to a Type 1 Seyfert galaxy a few years later before returning to its initial classification as a Type 1.9 Seyfert galaxy after 30 years (Huse-

<figure><img src="image_13.png" /><figcaption>Figure 12: X-ray spectra of ESO 323-G77 showing significant time-variable column density variations. Data from *Chandra* (2010), *XMM-Newton* (2006, 2013), and *Suzaku* (2011). When the obscuration is high, the Fe Kα line at 6.4 keV is most prominent. The column density varied between $10^{22} \text{ cm}^{-2}$ to $1.5 \times 10^{24} \text{ cm}^{-2}$. Source: reproduced from Fig. 1 of Miniutti et al. (2014).</figcaption></figure>
mann et al., 2016). It is in this phase, that a strong outburst was observed in 2020, as shown in Fig. 13 (Brogan et al., 2023). The Eddington ratio increased by a factor ≈ 13 from 0.004 to 0.052 during the outburst. The shape of the decline over the following ≈ 200 days is incompatible with a TDE, and therefore the event is most likely caused by a drastic, short-term increase in the accretion rate. Since Mrk 1018 is undergoing a galaxy merger, it is possible that these events are due to a binary SMBH or a recoiling SMBH. The first changing-look AGN “events” to be identified occurred in Seyfert galaxies (e.g. McElroy et al., 2016), but the first changing-look quasar was identified about decade ago (LaMassa et al., 2015). Through an SDSS and Pan-STARRS-1 survey, MacLeod et al. (2019) found 17 new changing-look quasars, yielding a changing-look rate larger than 20% among variable quasars (changes in optical continuum magnitudes by ≈ 0.5–1 mag), while the combination of the SDSS and DESI quasar surveys is now allowing for the systematic identification of changing-look quasars in sample sizes of > 500 (Guo et al., 2024b).

Simulations of changing-look AGN, have explained the observations through accretion-rate variations, but also from the stochastic character of the gas dynamics in the sub-pc region, see Fig. 14. Since in numerical simulations, there exist frequent variations due to the intermittent outflows around an AGN, Wada et al. (2023) propose that the time-scale of ~10 years is too short for a change of the accretion disk, but the changing look could come from changes in the structures of the emission line regions. Even without a direct change of accretion rate, it is possible that the stochasticity of the outflow phenomenon produces significant changes in the spectrum of the ionized gas, in particular the Balmer emission lines. In the simulation, the equivalent width of Hα and Hβ changed by a factor of 3 with the emission lines disappearing over 30 yr. These variations come from the gas dynamics within the dust sublimation radius. The AGN radiation pushes the dust and drives the gas outflow intermittently in the sub-pc region of the disk.

### 3.6. Extreme events and quasi periodic eruptions

Extreme quasar variability has been studied as flares superposed on their normal stochastic variability (Graham et al., 2017). These flares occur in $5 \times 10^{-5}$ of the million quasars considered and last typically 3 years. Micro-lensing by stars in foreground galaxies could be one explanation (Lawrence et al., 2016). But the majority could be due to a changing-look event, and explosive stellar-related activity in the accretion disk: super-luminous supernovae, TDEs, and mergers of stellar mass black holes. They can also be due to the interaction of a supermassive black hole with a lower mass companion in an extreme mass-ratio inspiral (EMRI) (Chakraborty et al., 2024), or with a single star (Linial and Metzger, 2023). TDEs can produce indeed spectacular variation, as in the Seyfert 2 galaxy GSN 069, where the X-ray emission increased by more than a factor of ≈ 240 in 16 years (Miniutti et al., 2019). After that, the decrease was slow over 8 years, with only a thermal spectrum. However, there were then X-ray quasi-periodic eruptions (QPE), with jumps by two orders of magnitude, over the course of 54 days on time scales of hours. Since GSN 069 is a

<figure><img src="image_14.png" /><figcaption>Figure 13: An outburst in the changing-look AGN Mrk 1018 shown at three wavelengths, the u'-band (blue: 300-400 nm), c-band (cyan: 420-650 nm), and o-band (orange: 560-820 nm). The u' and c-band data have been shifted vertically for visualisation purposes. The magnitude of the outburst is independent of the colour, implying that dust obscuration is not the cause. Source: reproduced from Fig. 9 of Brogan et al. (2023).</figcaption></figure>

dwarf galaxy with a $4 \times 10^5$ M$_\odot$ SMBH, the class of AGN with more massive SMBHs could have the same eruptions on time scales of months, given the relative relevant sizes. We should note that X-ray bursts are quite common in stellar-mass systems (Galloway et al., 2008), but difficult to identify in SMBHs, although they should also be quite common. A handful of QPE have been detected since 2019 in low-mass AGN-hosts (Arco dia et al., 2024). They are thought to be due to gas accretion from a white dwarf in a very eccentric orbit about the central massive black hole. The orbit of the white dwarf is progressively reduced in radius and eccentricity, due to radiation of gravitational waves (King, 2023a). The energy liberated at each passage is then much lower, while the phenomenon becomes a quasi-periodic oscillation, hardy detectable. The constraint of a closely orbiting star with high eccentricity favors the low-mass SMBH (King, 2023b).

To represent the observations of AGN X-ray and optical variability, Kubota and Done (2018) built a model of AGN including a standard thin disk in the outer parts, truncated towards the inner part, to contain a warm region to produce the soft X-ray excess and a hot corona, the source of hard X-rays. The size of the hard X-ray corona is equal to the radius where there is sufficient accretion energy to power the observed X-ray emission. Such a model succeeds in producing a harder X-ray spectrum towards lower Eddington ratios, reproduces the X-ray/UV correlation, and an increasing optical variability with decreasing Eddington ratio.

A changing look event can be dramatic, as shown by Ricci et al. (2020). In this example the Type 2 AGN 1ES 1927+654 experienced drastic changes in the X-ray band, with variations up to ~4 dex over ~100 days, with short time-scale variability up to ~2 dex in ~8 hrs. This was accompanied by a complete disappearance of the hot corona, and depletion of the whole inner disk. After several months the disk and hot corona were recreated. These extreme AGN variability events with factors of 2–10 optical/UV/X-ray luminosity changes over 1–10 yr time
scales suggest a change in our view of accretion disks. Indeed, with the standard thin accretion disk, the propagation time-scale is too long, of the order of 100 years, for a $10^6$ M$_{\odot}$ black hole; however, in a geometrically thick disk, the accretion flow will be shorter, on par with the variability timescale (Dexter and Begelman, 2019).

Variations are not only seen at X-ray/UV/optical wavelengths, but also in the mid-IR waveband, in more than a hundred sources selected from the WISE survey (Jiang et al., 2021). Only four galaxies were found to be radio loud, indicating that synchrotron radiation from relativistic jets is not the main cause of the variability. It is likely that these mid-IR outbursts are dominated by the dust echoes of transient accretion onto SMBH, either through TDEs or changing-look AGN. This is supported by the compatibility of the mid-IR peak luminosity with X-ray and optical TDE phenomena at the high end.

### 3.7. Simulations of cosmic SMBH growth

Over the last decade, there has been considerable progress in simulations of SMBH growth, in particular within the broader cosmological context (Vogelsberger et al., 2014b; Sijacki et al., 2015; Dubois et al., 2014a; Volonteri et al., 2016; Weinberger et al., 2017b; Davé et al., 2019), adopting special recipes to take into account of sub-grid physics, such as the Bondi accretion rate, or the repositioning of the SMBH, to oppose its artificial wandering. The repositioning is applied in simulations that lack a subgrid model for dynamical friction, which applies to the majority of existing simulations today. In some simulations, a drag force is implemented, exerted on the SMBH by the gas and/or stars and/or dark matter (Sijacki et al., 2011; Dubois et al., 2015; Tremmel et al., 2017; Chen et al., 2022a; Ni et al., 2022b). Fully modelling dynamical friction requires a spatial resolution finer than 10 pc (Pfister et al., 2019), which is beyond the capacities of most cosmological simulations. In parallel, at much smaller scales, it has been realized that the standard model of thin disks, for Eddington ratios over 0.01–0.2, were thermally unstable, and incompatible with observations (e.g. Davis and Tchekhovskoy, 2020). The advent of 3D-magneto hydrodynamical (MHD) simulations, taking into account of radiation and general relativity, have made great progress towards solving this problem. With high spatial and temporal resolution many of these simulations have been performed at the individual object scale, to follow the accretion flow close to the SMBH, including general relativity (or pseudo-Newtonian) and MHD, with radiative transfer. For sub-Eddington ratios, of 0.07–0.2, Jiang et al. (2019) finds departures from the standard model of a thin accretion disk, depending on the amount of magnetic pressure, which is key to thickening the accretion disk.

Several 3D-MHD simulations have shown that magnetic-pressure-dominated thin, sub-Eddington accretion disks can maintain thermal equilibrium, in contrast to thermally unstable disks dominated by radiation pressure (Sadowski, 2016). The magnetically supported thin disks are thicker than standard accretion disks at the same radius and accretion rate. These thicker disks are thought to be more compliant with observations, in particular when the observed X-ray/UV/optical variability is concerned. As discussed in the previous sub-sections,

the observed variations, when due to intrinsic variations of accretion rate, are too rapid to be explained within the standard model of a thin accretion disk, although thicker accretion disks may better explain the observations (Dexter and Begelman, 2019). Mishra et al. (2022) demonstrated that magnetically-dominated disks are thermally stable and that the magnetic fields can both ensure the stability of somewhat thicker disks, and drive accretion onto the central SMBH through magnetorotational instability (MRI). For that to occur the best configuration of the magnetic field is vertical, perpendicular to the accretion disk, where it is possible to generate a toroidal field near the mid plane of the disk.

While at lower Eddington ratios (0.01–0.07), the accretion flow is mainly through the optically thin corona, for higher accretion rate systems, the accretion disk surface density increases with radius. The turbulence driven by the MRI which transports the angular momentum away, fostering a rapid accretion flow. Super-Eddington accretion ($\lambda_{Edd} = 20$) onto a maximally rotating SMBH has been simulated by McKinney et al. (2014), resulting in the formation of a persistent radio jet through the Blandford-Znajek mechanism, and driving an accretion-disk wind. The energy loss from the wind reduced the accretion efficiency down to 20%, and the total radiative efficiency dropped down to 1%, implying an observed AGN luminosity equal to that expected for the Eddington limit.<fnref n="4" /> The accretion disk is quite geometrically thick, corresponding to the slim disk regime. This shows that it might be possible in the early Universe, to grow SMBHs quickly through super-Eddington accretion (see §5.4 for potential super-Eddington accretion at high redshift).

It is quite important to simulate also the galactic environment, to better understand how the gas replenishment occurs and allows for more or less continuous fuelling of the AGN. The required range of scales in such a simulation are huge, and hydrodynamical codes with refinement, and even hyper-refinement are required. Starting from zoom-in simulations, identifying a given halo in a cosmological context, Anglés-Alcázar et al. (2021) have then run an hydrodynamical simulation refined down to a resolution of 0.1 pc around the SMBH. They chose three epochs corresponding to a pre-AGN, a full luminous AGN (i.e., a quasar) and a late-time AGN, differing in gas mass, star formation rate (SFR), and SMBH accretion rate at $z \sim 2$. They found a quasi-steady state is reached, where the gas inflow at 0.1 pc scale is at least $6 M_{\odot} yr^{-1}$, in the full luminous AGN phase. This active phase is short ($\approx 2$ Myrs) and the duty cycle is brief, separated by long inter-cycles. Star formation consumes the bulk of the gas inflow, although supernovae feedback is also taken into account in the simulation, but not the impact of AGN feedback. The mechanisms driving the gas inflow are the $m = 2$ non-axisymmetries, primary and secondary,

<footnote n="4">⁴For the definition of efficiencies, let us note that not all of the mass accretion serves to grow the SMBH, first a component (typically ≈ 10%) is radiated away. At low Eddington ratios (< 0.01), part of the mass is advected into the SMBH, without radiation (increasing the accretion efficiency but lowering the radiation efficiency). Also part of the mass is lost to driving an outflow (i.e., a jet and/or a wind), further reducing the accretion efficiency.</footnote>
<figure><img src="image_15.png" /><figcaption>Figure 14: Hydrodynamical simulations of gas accretion onto an SMBH, illustrated at 3 epochs separated by just 30 years. The strong variability illustrates the intrinsic changing-look AGN phenomenon, through accretion rate variations. Top and bottom plots focus on the central 0.2 pc and 0.002 pc regions, respectively. The left-half panels show $V_z$ (top) and $V_{outflow}$ (bottom). The right-half panels show the gas number density. Source: adapted from Fig. 2 of Wada et al. (2023).</figcaption></figure>

and the gas at sub-pc scales remains unaligned with respect to the large-scale disk (see Fig. 6 and Fig. 8).

Following a cosmological halo identified at $z = 6$, Dubois et al. (2014b) studied the evolution and growth of a central SMBH, through gas accretion and galaxy mergers. Since the physics of SMBH accretion needs to be at the sub-grid scale in the simulation, they used semi-analytical recipes for AGN feeding and feedback. They found the SMBH can rapidly change its mass and spin throughout its lifetime of gas accretion and mergers. In the high-redshift universe, the gas is quite abundant in cosmic filaments and gas accretion dominates the SMBH growth, while galaxy mergers, and subsequent SMBH mergers, dominate at lower redshift. At high redshift, gas accretion has a more coherent angular momentum, which allows the spin of the SMBH to grow, while at lower redshifts, mergers of galaxies and their SMBHs have more random orientations, implying a reduced spin. Radiative efficiency is also higher early in the evolution, since it follows the spin amplitude. The resulting alignment between the SMBH spin and angular momentum of the host galaxy is then never completely achieved, but it is higher for intermediate mass galaxies; note, SMBH accretion disks and their galaxy hosts are randomly oriented in massive galaxies.

Large-scale cosmological simulations are essential for capturing the growth of the entire SMBH population, from those residing in dwarf galaxies to the largest ones, over cosmic time. These simulations, with box side lengths ranging from tens to hundreds of Mpc (Dubois et al., 2014a; Hirschmann et al., 2014; Genel et al., 2014; Vogelsberger et al., 2014b; Sijacki et al., 2015; Khandai et al., 2015; Volonteri et al., 2016; Pillepich et al., 2018; Weinberger et al., 2017b; Davé et al., 2019; Ni et al., 2022b), are primarily calibrated to reproduce

galaxy properties (e.g., stellar mass function, fraction of quiescent galaxies, sizes) and the SMBH to stellar mass ratios of local massive galaxies with $M_{\star} \ge 10^{10.5} M_{\odot}$ (Habouzit et al., 2021, 2022, for an overview). Since these simulations are not calibrated on other SMBH or AGN properties, they enable meaningful comparisons with observations. In particular, they produce a relatively good agreement with the SMBH mass function at $z = 0$, and produce the AGN downsizing, or anti-hierarchical AGN distribution with time; i.e., the number density of luminous AGN peaks at higher redshifts than those of faint AGN (Hirschmann et al., 2014). This is attributed to the higher gas abundance at high redshift (see §5). The mean of the logarithm of the Eddington ratios varies across simulations; however, all show an increase with redshift (Habouzit et al., 2022), consistent with observations (Kelly and Shen, 2013). The simulations also approximately reproduce the AGN luminosity function at $z = 0$ but encounter issues at the faint end for $z \ge 1$ where most of the simulations overpredict the number of AGN with $L_{bol} \le 10^{45} \text{ erg/s}$. The faint end is driven by a combination of the subgrid physics related to SMBH formation (i.e., seed mass), feedback from SNe and AGN, and the accretion of SMBHs, all of which are designed to address the limitations in resolution that hinder the self-consistent capture of these processes. The above subgrid models and their variations across simulations lead to different SMBH-galaxy mass correlations, particularly in low-mass galaxies. The lack of stochasticity in these models often results in tighter relations than observed in the local Universe (Reines and Volonteri, 2015, and Fig. 15). Some simulations produce a population of SMBHs that are overmassive relative to the mass of their host galaxies or halos; all due to tidal stripping of their hosts (Volonteri et al., 2016; Barber et al., 2016). In simulations without SMBH
repositioning, the high-mass end of haloes may even host several SMBHs, and when two are gas fuelled and radiate, this produces dual AGN hosted within the same galaxies (see also §4.5). This quite rare occurrence corresponds to the observations at low redshift.

AGN feedback via AGN-driven outflows is thought to be important in regulating the galaxy mass and producing the tight M-σ relation, but also in preventing the formation of galaxies that are over massive compared to those observed. Abundant matching across a range of cosmic epochs shows that the star formation in massive galaxies peaks early, before being quenched, and that this quenching is related to their SMBH growth (Moster et al., 2013). The right equilibrium between runaway star formation in massive galaxies and the ejection of most of the gas outside of galaxies is hard to fine tune. Towards this goal, Weinberger et al. (2017b) introduce a new model for the AGN feedback. For high-accretion rates, above a given threshold depending on SMBH mass and the Eddington ratio, a fraction of the mass energy of the accretion heats the gas, preventing star formation (see also §6). In the case of low accretion rates, they employed a kinetic feedback model designed to be more effective. This model pushes gas in random orientations, generating shock waves that heat the gas and subsequently quench star formation. This heating is more uniformly distributed, and has more impact on galaxies. Their cosmological simulations succeeded in producing the red and dead massive elliptical galaxies (Nelson et al., 2018), although with a population of SMBHs not fully aligned with observational constraints.

Zoom-in simulations, able to manage a large range of scales, are now common practice in the cosmology and galaxy domain. Care must be taken to adapt the approximations made to simulating a particular scale, to the next zoom-in scale of the simulation. In particular, the physics of the gas in the AGN circum-nuclear region is quite different from the gaseous disks forming stars at kpc scales. Feedback recipes are different, and the impact of the magnetic field or the radiation transfer have to be adapted to the scale. In their ambitious multi-scale enterprise, Hopkins et al. (2024) modified their simulations to modify the physics from cosmological scales to radiation-magnetohydrodynamic (RMHD) computations of accretion flow near a central SMBH. Their simulation spans scales from ~100 Mpc down to ~100 au to feed a bright quasar, with accretion rates as high as 10–100 M$_{\odot}$ yr$^{-1}$. The gas infall is driven by gravitational torques from stellar bars and spirals down to 1 pc scales. The physics at sub-pc scales changes, with a high fraction of ionized gas and strong magnetic fields. In the accretion disk, the gas is turbulent, no longer self-gravitating, and subject to the MRI instability. The star formation, is also efficiently suppressed. An aspect of the multiphase gas over this huge range of scales (accretion disk, BLR, torus, host galaxy, and halo) is displayed in Fig. 1.

### 3.8. Summary of the key drivers of progress

Looking back over the last decade since AH12, huge progress has been made in the domain of AGN fuelling and SMBH growth, due both to new observational facilities, great

advances in the telescope sensitivity and spatial resolution, and also theoretical and conceptual advances. On the observational data side, the last decade has seen the advent of ALMA, yielding for nearby AGN about 10 mas resolution, resolving the radius of gravitational influence of the SMBH ($R_{\text{Rol}}$). Since 2018, the VLTI with MATISSE and GRAVITY have provided 5–10 mas resolution in the near and mid IR wavebands, and a conceptual breakthrough has been made in mapping circumnuclear regions of AGN, identifying polar dust cones. Thanks to the high resolution of the VLT interferometer, GRAVITY Collaboration et al. (2024) has measured the BLR sizes in some AGN, and found them compatible to that determined through reverberation mapping; see also §5.1. The EHT has achieved a record-breaking 20 µas in resolution at millimetre wavelengths, allowing us to witness the shadow of two SMBHs, and the launching of the radio jets. Since 2022, JWST has brought a wealth of data on the dust and gas surrounding the SMBH, with a huge amount of excited lines, together with the H$_2$ rotational ladder, and the PAH. As described in §5.4, one of the main surprises from JWST is the potential over-massive SMBHs from candidate AGN identified in the early universe, with respect to their host stellar mass. Super Eddington accretion (e.g. Madau, 2025) may help reduce the black hole mass estimates, which are quite uncertain (Lupi et al., 2024a,b).

On the theoretical side, numerical simulations have been flourishing, with cosmological runs following the SMBH growth across cosmic times, and GR-MHD codes, with radiative transfer in 3D, which are transforming our vision of the accretion disks, and their different regimes, as a function of the Eddington ratio. The huge advances were due to several factors: not only the large increase in computing power, allowing higher resolution and larger volumes and, therefore, greater statistical power, but also more detailed and refined physics, with new algorithms and techniques.

The last decade has also seen great advances in monitoring AGN variability thanks to new facilities such as ZTF and ASAS-SN which provide large-area optical photometric imaging to allow for the identification of transient phenomena and to track small-amplitude variations in the output of objects in the time domain. This has allowed for the first large-scale searches for TDEs and the monitoring of changing-look AGN. These developments have helped to show that differences between Type 1 and Type 2 AGN cannot be just explained by the unified AGN model, accretion rate is also a key factor in determining the observed properties. In the future, more sensitive time-domain facilities such as the Vera Rubin telescope will further develop this research field, opening up time-domain astronomy to a larger fraction of the research community.

## 4. What properties of the host galaxies or larger-scale environment affect black-hole growth?

The previous section reviewed progress in our understanding of the mechanisms that brings gas to the centres of galaxies to fuel the accretion process and provided an overview of accretion onto SMBHs. In this section we discuss what we have
learnt about whether these processes preferentially occur—and thus trigger AGN—within galaxies with certain properties or within certain large-scale environments. Such studies can help us understand how SMBH growth and the growth of galaxies proceed together over cosmic time as well as revealing whether AGN phases and the huge energy they release in different forms has an *impact* on the properties and evolution of galaxies or their larger scale environment (such “feedback” processes are discussed further in §6).

Two key results that indicate the existence of an *overall* connection between the growth of galaxies and the black holes that lie at their centres have been well established now for a number of decades. First, the SFR density and the black hole accretion rate density are found to follow a similar pattern over cosmic time, both peaking when the Universe was around 2–6 Gyr in age (z ~ 3 to z ~ 1, referred to as “cosmic noon”)<fnref n="5" /> and dropping by a factor of ~ 10 thereafter (e.g., Boyle and Terlevich, 1998; Madau and Dickinson, 2014; Aird et al., 2015b). Thus the total amount of galaxy stellar growth and the total black hole growth via accretion appear to be correlated, suggesting a common origin. Second, the masses of *inactive* black holes at the centres of galaxies in the local (z ~ 0) Universe are found to be correlated with a number of different, larger-scale galaxy properties—most tightly with the velocity dispersion of the central galaxy bulge, producing the famous “$M_{BH} - \sigma$” relation (Ferrarese and Merritt, 2000; Gebhardt et al., 2000; Tremaine et al., 2002). Correlations are also found with the bulge luminosity or mass (e.g. Magorrian et al., 1998; Marconi and Hunt, 2003; Häring and Rix, 2004) and the total stellar mass (e.g. Cisternas et al., 2011a; Reines and Volonteri, 2015, see Fig. 15, left panel), albeit with larger scatter and possible dependencies on other galaxy properties such as SFR (e.g., Terrazas et al., 2017; Greene et al., 2020). Direct, dynamical SMBH mass measurements only remain possible for a small number of nearby *inactive* galaxies, where the SMBH sphere of influence can be resolved (see review by Kormendy and Ho, 2013). However, extensive efforts over the last decade have extended SMBH scaling relations to include both broad-line AGN (see §5.1 for further discussion) and obscured AGN (e.g. Baron and Ménard, 2019) and to push constraints to lower SMBH and galaxy masses in the dwarf galaxy regime (see reviews by Reines and Comastri, 2016; Greene et al., 2020). The exact form of these scaling relations and the origin/implications of the scatter remain an area of active research (e.g. Shankar et al., 2019; Habouzit et al., 2021; Terrazas et al., 2024). Nonetheless, these relations indicate a broad connection between the build up of the black hole mass (i.e. the result of past AGN activity within a galaxy) and the build up of the stellar component, showing that a connection between black hole and galaxy assembly may apply to *individual* galaxies.

Much research effort over the last decade has focused on trying to track this relationship as *the build up of both galaxies*

and *their* SMBHs *occurs* across different epochs of cosmic time. The last decade has seen an advance in the *data* available to identify AGN and their hosts due to both the culmination of major survey efforts with the premier facilities (e.g. *Chandra*, *XMM-Newton*, *GALEX*, *HST*, *Spitzer*) and a range of new facilities that provide unprecedented sensitivity or sky coverage at key wavelengths (e.g. *NuSTAR* at hard X-ray energies, *VISTA* in the near-IR, *WISE* in the mid-IR, *Herschel* in the far-IR, *LOFAR* at radio wavelengths). Alongside, there have been crucial developments in the *techniques* to disentangle the emission from AGN and their host galaxies and thus enable accurate measurements of the galaxy properties. We provide a succinct review of the progress due to both these elements in §4.1 below. A result of such work has been a major *conceptual* development in our understanding of SMBH growth as a transient, varying process throughout the lifetime of a galaxy, which is discussed in §4.2. In §4.3 we focus on a different mode of SMBH activity—associated with strong relativistic jets that are most clearly revealed by radio surveys—that is found to be distributed differently across the galaxy population and appears to play a role in capping both the growth of a galaxy and its central SMBH. An interesting development that has followed from our new understanding of how AGN (of differing types) occupy galaxies has been to revise our understanding of the relationship between large-scale environment and AGN activity, indicating (for the most part) that any such connection may be secondary to the more direct connection with the host galaxy properties (see §4.4). In §4.5 we aim to clarify an issue of much debate over the past decade: the role that mergers between galaxies play in the triggering of AGN activity while in §4.6 we discuss how a new field of research has emerged over the last decade focusing on active massive black holes in dwarf galaxies (i.e., galaxies with low stellar masses). Finally, in §4.7 we reflect on the key drivers that have led to progress in this field.

## 4.1. *Identification of AGN and measurements of galaxy properties*

A primary feature of AGN is emission across the electromagnetic spectrum, and thus a wide variety of identification techniques at different wavelengths are well established (see Padovani et al., 2017; Hickox and Alexander, 2018, for systematic reviews). All methods of identifying AGN produce incomplete samples due to both the diversity of intrinsic AGN properties (e.g., accretion rate, black hole mass, obscuration/absorption by different components, strength of any collimated jet) and the relative strength of light from the host galaxy that can make it difficult to distinguish emission due to an AGN from that produced by other processes (e.g., ongoing star formation, existing stellar populations, hot interstellar gas). Ultimately, all methods of AGN selection require choices to be made and will thus be incomplete in terms of their sampling of the underlying *SMBH* population.<fnref n="6" /> One of the major advances

<footnote n="5">⁵We note that *JWST* discoveries of surprisingly large populations of candidate AGN at higher redshifts (see §5.4) indicate substantial BH mass assembly may occur at early cosmic times and *precede* the bulk of galaxy stellar assembly (see e.g. Yang et al., 2023a; Akins et al., 2024).</footnote>

<footnote n="6">⁶A related difficulty when selecting samples is determining when an active SMBH should be considered an AGN. A threshold may be chosen in terms of luminosity (at a given wavelength). Alternatively, an Eddington rate threshold</footnote>
<figure><img src="image_16.png" /><figcaption>**Figure 15:** Left panel: SMBH mass as a function of the total stellar mass of their host galaxies in the local Universe, demonstrating the overall connection between the growth of galaxies and their central SMBHs. The sources are categorized into several groups: SMBHs with dynamical mass measurements in large galaxies (light blue) and spiral galaxies (dark blue), reverberation-mapped AGN (purple), and broad-line AGN (teal), all presented in Reines and Volonteri (2015); Davis et al. (2018), and SMBHs in low-mass and dwarf galaxies (pink to red symbols, including several upper limits, Reines and Volonteri, 2015; Greene et al., 2020), and SMBHs in BCG galaxies (green, Bogdán et al., 2018). The contours represent the combined populations of these groups. Additionally, we include a large sample of obscured AGN from SDSS in the background ($z \le 0.3$, grey, Baron and Ménard, 2019). The solid black line shows $M_{BH} = 0.002 M_{star, total}$ (Marconi and Hunt, 2003) that is widely adopted in definitions of specific SMBH accretion rates or kinetic power (see footnotes 12 and 22). Right panel: Sources at high redshift ($z \ge 4$) discussed in §5.4. Quasars characterized with JWST observations, i.e. with both SMBH mass and total stellar mass measurements, are represented by pink and purple symbols (Ding et al., 2023; Stone et al., 2023; Yue et al., 2024c; Marshall et al., 2024). JWST-discovered AGN candidates with stellar mass estimates are shown in orange (Kocevski et al., 2023; Harikane et al., 2023; Maiolino et al., 2023; Ono et al., 2023; Larson et al., 2023). The contours represent systems in the local Universe and are shown to guide the eye.</figcaption></figure>

ments over the last decade has been to improve our understanding of these incompletenesses, primarily by adopting multi-wavelength approaches to determine the limitations of different AGN selection techniques (e.g. Goulding et al., 2014; Mendez et al., 2013; Azadi et al., 2017; Padovani et al., 2017; Hickox and Alexander, 2018; LaMassa et al., 2019; Georgakakis et al., 2020). Here we describe some of the most widely adopted approaches to identify AGN and the techniques used to measure their host galaxy properties, with a focus on new developments over the past decade.

### 4.1.1. Optical/UV/emission-line selection

Selection of AGN based on their optical/UV properties provides arguably the most direct tracer of SMBH growth, as the accretion disk emission peaks at UV wavelengths and has a blue continuum that extends into the optical regime (see Fig. 2). However, such selection is particularly sensitive to the impact

of host galaxy dilution – weaker AGN may easily be overwhelmed by the emission from stars at these wavelengths – as well as obscuration effects as relatively small amounts of dust or gas in the line-of-sight (within the host galaxy or associated with the AGN system itself) will drastically redden or completely extinguish the AGN light. Nonetheless, selection on the basis of blue optical colours combined with spectroscopic confirmation (primarily via the presence of broad emission lines, verifying the presence of a “Type-1” AGN) has been widely exploited as a means of identifying large samples of luminous and unobscured quasars. These quasars evolve strongly over cosmic time, peaking in space density at $z \sim 2$, although recent studies find only a weak “downsizing” whereby the space density of lower luminosity sources peaks later in cosmic time (i.e. at lower redshift) than higher luminosity sources (e.g. Ross et al., 2013; Kulkarni et al., 2019; Schindler et al., 2019, cf. the stronger downsizing signature seen for X-ray selected AGN discussed in §4.1.2 below). The properties of the most massive and most rapidly accreting examples of such quasars are explored in more detail in §5. The presence of broad emission lines may be used more generally as an indicator of AGN in blanket spectroscopic follow-up programmes (e.g. Driver et al., 2009; Hahn et al., 2023) including in sources that lack a blue optical continuum (e.g. due to dust reddening) but are selected by other means (e.g. Glikman et al., 2004; Banerji et al., 2012, see also §5.4 for discussion of the spectroscopic identification of high-redshift AGN candidates in “little red dots” using JWST).

<footnote>(i.e. relative to the SMBH mass) may correspond to a more fundamental parameter, reflecting the relative growth rate of the SMBH mass, but is difficult to measure (requiring accurate SMBH mass measurements) and introduces an issue for lower mass systems as a given Eddington rate will produce a lower absolute luminosity that may preclude detection. Another route is to consider the luminosity produced by the AGN relative to the emission due to host galaxy processes (in particular star formation) at the same wavelength. While such a selection can produce a robust sample of definitive AGN, such choices will exclude intrinsically weaker AGN as well as AGN that lie in more luminous galaxies (e.g. due to higher SFRs) which overwhelm the AGN signatures.</footnote>
A more recent area of development has been to use optical variability as a means to identify the presence of low-luminosity AGN even when the bulk of the optical light is dominated by the host galaxy (e.g. De Cicco et al., 2015, 2019; Sánchez-Sáez et al., 2019; Pouliasis et al., 2019), addressing some of the issues with host galaxy dilution. Such techniques are likely to be revolutionized in the near future by the LSST with the Vera Rubin Observatory (Ivezić et al., 2019). Isolating blue, quasar-like light from galactic centres using high spatial resolution imaging (e.g. with JWST or Euclid) may also provide a route for the identification of weak optical/UV AGN within luminous host galaxies (e.g. Euclid Collaboration et al., 2025a,b).

The strong UV emission produced by an accretion disk also photo-ionises gas clouds at distances up to ~kpc scales, resulting in characteristic narrow emission lines that are seen even when the direct line-of-sight to the central engine is blocked by obscuring material, producing a “Type-2” AGN. Emission-line ratio diagnostics that use the strongest optical emission lines (e.g. Hα, Hβ, [O III], [N II], [S II]) to identify AGN and distinguish them from star-forming systems are well established and had already been widely applied to provide a detailed picture of AGN activity within the local (z ≲ 0.3) galaxy population at the time of the AH12 review (see section 3.2 therein, see also Heckman and Best, 2014). However, the last decade has seen a greater understanding of the contribution of stellar processes to these emission lines that can dilute the signatures of lower luminosity AGN, particularly in star-forming galaxies, and has led to a re-assessment and re-interpretation of many of these results (e.g. Trump et al., 2015; Jones et al., 2016; Agostino and Salim, 2019, see §4.2 below). More extensive IFU spectroscopy has also enabled a spatially resolved picture that can aid in the separation of galaxy-wide star formation and central AGN activity (e.g. Wylezalek et al., 2018; Comerford et al., 2022). Furthermore, new facilities that enable extensive multi-object spectroscopy in the near-to-mid IR (e.g. Keck-MOSFIRE, VLT-KMOS, JWST/NIRSpec) have allowed these rest-frame optical diagnostics to be applied to statistical samples of distant, higher redshift galaxies (e.g. Coil et al., 2015; Harrison et al., 2016a). While such studies are effective in revealing AGN activity at these earlier cosmic epochs (including AGN that may not be identifiable via other techniques), the higher SFRs, lower metallicities and younger stellar populations of high-redshift galaxies can also produce strong emission lines that leads to greater dilution of the AGN signatures by the host galaxy light – an issue that appears to impact JWST spectroscopic studies that are now enabling rest-frame optical/UV emission-line diagnostics out to even higher redshifts (e.g. Scholtz et al., 2023, see §5.4). Star formation dilution may be mitigated by using fainter, higher ionisation potential lines found at rest-frame UV, optical or IR wavelengths that require the strong ionising continuum of an AGN (e.g. Feltre et al., 2016; Calabrò et al., 2023). IR emission lines have the added benefit of being less affected by dust extinction; however, such lines are typically weak and current IR spectrographs have much more limited multi-plexing than optical spectrographs, making it more challenging to obtain sensitive IR spectroscopy for large samples.

### 4.1.2. X-ray selection

X-ray emission, primarily produced by inverse Compton-scattering of UV photons from the accretion disk by a hot, compact corona (extending to ≲ a few $r_s$, e.g. Fabian et al., 2015), provides an alternative tracer of the AGN accretion power and an efficient means of identifying AGN (see Brandt and Alexander, 2015; Kara and García, 2025, for reviews); see Figs. 1 & 2. A major advantage of X-ray selection of AGN is that other galactic sources of X-rays (such as the population of X-ray binaries throughout the galaxy or hot gas) are relatively weak, resulting in a strong host-to-AGN contrast. Furthermore, higher energy X-rays (≥ 10 keV) are relatively immune to photoelectric absorption (up to equivalent hydrogen column densities, $N_H \sim 10^{24} \text{ cm}^{-2}$) while the absorption of softer X-rays produces a clear spectral signature that enables measurements of the column density. At higher column densities ($N_H \gtrsim 1.5 \times 10^{24} \text{ cm}^{-2}$), corresponding to the Compton-thick regime, X-rays of all energies are no longer able to penetrate without being Compton scattered, which substantially suppresses the directly transmitted spectrum, while some photons may be scattered back into the line of the sight with an altered spectrum with a characteristic “Compton hump” peaking at rest-frame energies ~ 30 keV (see Fig. 2). The need for a substantial population of Compton-thick AGN (~ 30–50% of moderate-luminosity AGN) to explain the peak in the overall cosmic X-ray background (CXB) at ~ 20–40 keV has been well established (e.g. Comastri et al., 1995; Gilli et al., 2007; Ananna et al., 2020). However, an area of significant progress has been the development of models to describe the absorption, line fluorescence, and scattering of X-rays by a realistic obscurer, expanding earlier work adopting toroidal geometries (e.g. Murphy and Yaqoob, 2009; Ikeda et al., 2009; Brightman and Nandra, 2011) to now include clumpy, varying and dynamic distributions (e.g. Tanimoto et al., 2020; Buchner et al., 2019, 2021) that align with our modern picture of accreting SMBH systems (see §3).

Given the relative immunity to obscuration, selection using hard (≥10 keV) X-rays can provide a comprehensive sampling of AGN. The *Neil Gehrels Swift Observatory* (Swift, launched 2004, Gehrels et al., 2004) and the *International Gamma-Ray Astrophysics Laboratory* (INTEGRAL, launched 2002, Winkler et al., 2003) were both primarily designed to identify and study Gamma-ray bursts; however, both contain wide-field hard X-ray instruments that provide all-sky survey coverage at > 15 keV energies (INTEGRAL-IBIS, Ubertini et al. 2003; Swift-BAT, Barthelmy et al. 2005) enabling surveys to find AGN. The ongoing missions have enabled progressively deeper surveys to be built up over the last decade and have identified samples of AGN (primarily z ≲ 0.2) of sufficient size (≥ 1000 sources) for robust statistical analysis of the population (Krivonos et al., 2022; Oh et al., 2018; Lien et al., 2023, see Fig. 16). A major effort to obtain optical spectroscopy for the *Swift-BAT* sources over the past decade (the BAT AGN Spectroscopic Survey, BASS: Koss et al., 2017, 2022; Oh et al., 2022) as well as other supporting multiwavelength data has enabled a robust characterization of the physical properties of the local AGN population.
<figure><img src="image_17.png" /></figure>

Figure 16: Coverage of the rest-frame 2-10 keV luminosity–redshift plane obtained through X-ray selection of AGN with different observatories that are sensitive to different observed-frame energy bands (as indicated in legend). Blue crosses show (non-beamed) sources from the *Swift*-BAT 158-month sample (Lien et al., 2023) that provides an all-sky survey at 14-195 keV. Orange crosses indicate sources selected at 8–24 keV compiled from a range of *NuSTAR* surveys (Mullaney et al., 2015; Civano et al., 2015; Masini et al., 2018; Zhao et al., 2024; Greenwell et al., 2024b). Red points compile data from a number of deep and wide *Chandra* and *XMM-Newton* surveys (Luo et al., 2017; Xue et al., 2016; Nandra et al., 2015; Marchesi et al., 2016; Masini et al., 2020; Chen et al., 2018; Ni et al., 2021a; Peca et al., 2024). Small green points show the sampling achieved in the ~ 140 deg² *eROSITA* Final Equatorial Depth Survey (eFEDS Brunner et al., 2022; Salvato et al., 2022) which already provides sample sizes comparable to prior efforts (primarily probing soft energies, 0.2–2.3 keV), while the green dashed line corresponds to the flux limit of the first *eROSITA* all-sky survey (eRASS1, Merloni et al., 2024) that promises unprecedented statistics for future studies of the X-ray AGN population. The horizontal grey lines indicate the intrinsic rest-frame 2–10 keV luminosities of two nearby AGN, the quasar PDS 456 and the heavily obscured AGN NGC 1068.

tion, including placing constraints on the overall $N_H$ distribu-
tion and Compton-thick fraction in the local Universe (Ricci
et al., 2017b), constraining the distribution of Eddington ratios
(Ananna et al., 2022b), revising our understanding of the im-
pact of radiation pressure on the structure and covering factors
of the putative AGN “torus” (Ricci et al., 2017c, 2022, 2023;
Ichikawa et al., 2019; Ananna et al., 2022a), and setting a local
baseline for the connection between galaxy mergers and lumi-
nous AGN activity (Koss et al., 2018, see also §4.5).

Launched in 2012, the *Nuclear Spectroscopic Telescope Array* (NuSTAR, Harrison et al., 2013) has provided ~2 orders of magnitude improvement in sensitivity at hard (≥ 10 keV) X-ray energies that enables the identification of AGN out to z ~ 3 (e.g. Aird et al., 2015a; Greenwell et al., 2024b), drastically expanding our sampling of the X-ray AGN population compared to the non-focusing hard X-ray telescopes discussed above (see Fig. 16). NuSTAR has confirmed the increasing importance of obscured AGN populations out to early cosmic times (Alexander et al., 2013; Aird et al., 2015a; Del Moro et al., 2017; Lansbury et al., 2017a; Zappacosta et al., 2018; Zhao et al., 2024) and directly resolved ~35% of the emission of the CXB at 8–24 keV (close to the peak) into individual AGN (Harrison et al., 2016b). However, NuSTAR surveys still remain sensitive to only the brightest sources and the sample sizes remain relatively small (≤2000 sources, with the vast majority identified serendipitously when targetting known sources: Lansbury et al., 2017b; Greenwell et al., 2024b).

Our most extensive census of X-ray AGN has instead been achieved with surveys at lower energies (~0.5–10 keV) thanks to the culmination of major deep and wide survey efforts with the flagship observatories, *Chandra* and *XMM-Newton*, that have the sensitivity to efficiently identify and characterize lower luminosity and obscured AGN populations out to $z \gtrsim 1$ and thus track the bulk of the AGN population (e.g. Nandra et al., 2015; Civano et al., 2016; Pierre et al., 2016; LaMassa et al., 2016; Luo et al., 2017; Chen et al., 2018; Masini et al., 2020; Ni et al., 2021a, see Fig. 16). Combined with extensive multiwavelength imaging and spectroscopic follow-up campaigns to obtain redshifts, these surveys confirm a strong “downsizing” evolution, whereby the space density of the most luminous X-ray AGN is found to peak at $z \sim 2$ whereas lower luminosity AGN peak in space density at progressively lower redshifts (see Fig. 17) as well as revealing a luminosity-dependent fraction of absorbed AGN that increases toward higher redshifts $^7$ (e.g. Ueda et al., 2014; Aird et al., 2015b; Buchner et al., 2015; Peca et al., 2023).

While *Chandra* and *XMM-Newton* have been effective in
covering small-to-moderate sky areas (~0.1-100 deg²) and
yielding the highest AGN sky densities (≥ 10⁴ deg⁻²),
*eROSITA* (launched in 2019) has recently provided a new all-
sky X-ray survey, yielding ~900,000 point sources (primarily

<footnote n="7">⁷Our current understanding of the underlying causes of both the downsizing behaviour and evolution of the absorbed fraction are discussed further in §4.2.</footnote>
<figure><img src="image_18.png" /><figcaption>Figure 17: Measurements of the space density of X-ray selected AGN of different ranges of luminosity (as indicated by the different colours), demonstrating the “downsizing” evolutionary behavior whereby lower luminosity AGN peak in space density later in cosmic time (i.e. at lower redshift) compared to high luminosity AGN. Source: reproduced from Fig. 18 of Peca et al. (2023).</figcaption></figure>

associated with AGN) in the first half-sky data release (Merloni et al., 2024), offering unparalleled statistics for future studies of the X-ray AGN population. The vast majority of these sources are detected at soft energies (0.2–2.3 keV) and are associated with luminous and unobscured AGN populations (Liu et al., 2022) spanning redshifts out to $z \sim 6$ (Wolf et al., 2021, 2024). In contrast, the sub-sample of $\sim 5500$ AGN that are detected in the harder, 2.3–5 keV band reaches $\gtrsim$ an order of magnitude fainter in X-ray luminosity than the Swift-BAT samples (Waddell et al., 2024a; Nandra et al., 2024), contains a higher proportion of obscured sources than the soft-selected samples and is helping reveal the complex structure of the obscuring material across the AGN population (Waddell et al., 2024b).

### 4.1.3. Infrared selection

The dusty, molecular gas that surrounds accreting SMBHs will absorb optical/UV light, which is then re-emitted at IR wavelengths with the emission typically peaking at around 15–60 µm (Mullaney et al., 2011), reflecting the typical temperature of the dust (~100 K). Detailed studies of nearby AGN systems over the last decade, exploiting the spatial information provided by high-resolution mid-IR observations using large ground-based 8–10m telescopes (e.g. Hönig et al., 2013; Asmus et al., 2016; Asmus, 2019; Leftley et al., 2021; García-Bernete et al., 2022), have shown that dust with a range of temperatures is typically present in a single AGN, with hotter dust likely associated with an outflowing wind extending in the polar direction, while cooler dust is likely associated with an inflowing, clumpy molecular gas disk that is also seen at sub-mm wavelengths (see §3.3 and Fig. 10 above).

The identification of AGN on the basis of a rising continuum toward longer wavelengths in the mid-IR, tracing the heated dusty material, was well established based on extensive surveys with the Spitzer telescope (e.g. Lacy et al., 2004; Stern et al., 2005; Donley et al., 2005, 2008, 2012; Alonso-Herrero et al.,

2006). The launch of WISE in 2009 (Wright et al., 2010) and subsequent release of all-sky imaging at 3.4, 4.6, 12, and 22µm (Cutri et al., 2013; Schlafly et al., 2019) has enabled such approaches to be extended to much larger datasets, revealing up to ~3 million candidate AGN (e.g. Assef et al., 2013, 2018; Stern et al., 2012; Mateos et al., 2012). WISE-selected AGN have been particularly useful when very large samples are required for statistical measurements (e.g. clustering analyses, see §4.4 below) or to provide reliable identification of counterparts and SED constraints for AGN identified at other wavelengths (e.g. Salvato et al., 2018, 2022; Auge et al., 2023).

IR identification is immune to obscuration effects (tracing emission from the obscuring dust itself) and as such is able to identify heavily obscured populations that are missed by other selections (e.g. Del Moro et al., 2016; Carroll et al., 2021; Anonie et al., 2022). It can also provide a reliable tracer of the accretion power (e.g. Gandhi et al., 2009; Mullaney et al., 2011). However, dust heated by recent star formation or circumstellar dust around older stellar populations can produce substantial mid-IR emission that can dilute the signal of intrinsically weaker AGN. Careful, cross-community work over the last decade comparing IR selections with AGN identified at other wavelengths has shown that IR selection is effective at selecting AGN that are highly luminous relative to their hosts but is biased against the selection of lower luminosity AGN in galaxies with high SFRs or with the highest stellar masses (Mendez et al., 2013; Azadi et al., 2017; Ji et al., 2022). Studies of the demographics also find a downsizing behaviour (i.e. lower luminosity AGN peaking in space density at lower redshift than higher luminosity sources; see e.g. Assef et al., 2011; Lacy et al., 2015) that is broadly consistent with findings at X-ray wavelengths (cf. Fig. 17 and §4.1.2).

More recently, new imaging obtained with the Mid-Infrared Instrument (MIRI) on JWST (Rieke et al., 2015) provides more than an order of magnitude improvement in mid-IR sensitivity compared to Spitzer, albeit over a small field-of-view (74 arcsec × 113 arcsec) that has meant overall survey coverage is limited to $\lesssim 300$ arcmin² (e.g. Yang et al., 2023b; Alberts et al., 2024; Leung et al., 2024). Initial studies have used SED-fitting to reveal populations of AGN including low-luminosity sources (in correspondingly faint/low-mass host galaxies) and likely obscured AGN that were unidentifiable with prior facilities (e.g. Yang et al., 2023a; Chien et al., 2024; Kim et al., 2024b; Li et al., 2024c; Lyu et al., 2024). Selection biases related to the distinction between host galaxy and AGN emission are if anything more acute in this regime compared to prior Spitzer or WISE studies (Kirkpatrick et al., 2023). These fainter/obscured populations do not significantly increase estimates of the total black hole mass accretion density, at least to $z \sim 3$, and thus do not drastically alter our understanding of AGN demographics<fnref n="8" /> (Yang et al., 2023a). Mid-IR spectroscopic diagnostics, which have been shown with Spitzer to be highly effective as a means of identifying highly complete AGN samples and miti-

<footnote n="8">⁸JWST discoveries of substantial AGN populations at $z \gtrsim 3$ with unexpectedly high space densities, including the “little red dots” found based on red near-IR colours, are discussed further in §5.4.</footnote>
gating both obscuration and host galaxy biases (e.g. Goulding and Alexander, 2009; Alonso-Herrero et al., 2012; Del Moro et al., 2016; Feltre et al., 2023; Bierschenk et al., 2024, see Sajina et al. 2022 for a review), are only just starting to be exploited with *JWST* (e.g. Armus et al., 2023; Goold et al., 2024).

### 4.1.4. Radio selection

For deep extragalactic surveys, the prime means for identifying AGN in the radio band is synchrotron emission from relativistic particles accelerating in magnetic fields. When referring to radio AGN, often researchers focus on the canonical Fanaroff-Riley type 1 / type 2 sources (Fanaroff and Riley, 1974), where the synchrotron emission is produced in powerful radio jets that range from kpc to Mpc in scale, far beyond the extent of their host galaxies. These types of AGN were historically defined as ‘radio-loud’ as they could be easily identified with a ratio of 5 GHz to B-band luminosity > 10 (Kellermann et al., 1989); those with a ratio < 10 were classified as ‘radio-quiet’ and the radio emission was thought to be dominated by star formation. With more sensitive radio surveys it has become clear that, while more than one population is required to make up the complete range of observed sources, there is a large overlap region between ‘radio-quiet’ and ‘radio-loud’ AGN (Fig. 15 in White et al., 2007).

To pick out “clean” samples of AGN hosting radio jets, often a single cut in radio loudness, or a luminosity cut that gives effectively the same selection (i.e., samples selected from flux-limited surveys with a small redshift range and similar host galaxy B-band luminosities), is used. However, while a high enough cut in radio loudness can provide a clean jetted-AGN sample, it also provides a biased picture. The fraction of galaxies containing radio-loud AGN increases steeply as a function of stellar mass (Sabater et al., 2019). Radio AGN also tend to be strongly clustered, living in dense large-scale environments (Magliocchetti et al., 2004; Hickox et al., 2009; Mandelbaum et al., 2009). The radio selection of AGN also often picks up radiatively inefficient AGN, which are thought to be fed by advection dominated accretion flows (ADAFs; e.g. Narayan and Yi, 1995) rather than the standard radiatively efficient accretion disk (Shakura and Sunyaev, 1973, see also §3.4 and Fig. 11). These ‘hot mode’ accretors show no evidence for an optically thick accretion disk (Hardcastle et al., 2007) and the bulk of their energy is contained in radio-loud jets (e.g. Merloni and Heinz, 2007). They will therefore not be selected as AGN at any other waveband. While initial studies demonstrated that radiatively inefficient accretion was linked to lower accretion rates (e.g. Best and Heckman, 2012), newer radio surveys find a broader range of accretion rates associated with the different AGN classifications (Whittam et al., 2022).

In galaxies where it is not clear that a powerful radio jet is present (generally referred to as ‘radio-quiet’), the source of radio emission is still a topic of debate. A recent review succinctly lays out the different options (Panessa et al., 2019): star formation, shocks from AGN winds, low-power jets, and coronal effects. Coronal emission, while possible, is only likely to be fruitfully investigated at mm frequencies (Behar et al., 2015) and have only a small impact in the regime where syn-

chrotron radiation dominates (≈ 10 GHz). The debate of star-formation vs. AGN activity as the origin of radio emission has supporters in both camps, and arguments usually rely on defining sources as radio excess above what would be expected for star formation as measured from another waveband, usually at far-IR wavelengths (Delvecchio et al., 2017; Best et al., 2023).

It is difficult to distinguish between synchrotron emission from star formation and AGN activity. Brightness temperature measurements can pick out AGN cores independent of using a separate measure of radio excess, given high enough resolution (Condon et al., 1991). Using milli-arcsec resolution with wide-field VLBI techniques at GHz frequencies is now relatively developed (Garrett et al., 2001; Herrera Ruiz et al., 2017; Radcliffe et al., 2018), but still limited in field of view and number of sources, which makes it difficult to place these radio-selected AGN within the context of the general AGN population. Recent work using brightness temperature measurements from high-resolution LOFAR imaging, which increases field of view and sample sizes by over two orders of magnitude, has shown that even in non-radio excess sources, there can be a significant fraction (up to 50% more than expected) of radio emission contribution from AGN activity (Morabito et al., 2022b, 2025). This also becomes evident when forward-modelling the radio population in the LoTSS Deep Fields assuming a two-component model: star formation + AGN activity. Macfarlane et al. (2021) demonstrated that this two component model, where the AGN population is drawn from a power-law distribution down to low radio power, can completely reproduce the source counts and radio-loud fraction in the observations. What is also revealed is that there is an entire population of sources which would be classified as ‘radio-quiet’ but have more than half of their radio emission generated by AGN activity (White et al., 2017; Macfarlane et al., 2021, see their Fig. 12). The models are agnostic to whether the AGN radio emission is generated by a jet (as assumed by this single component model) as opposed to other radio AGN emitting processes, which allows for a new way to explore the radio properties of different AGN sub samples (Yue et al., 2024a).

### 4.1.5. Galaxy properties

Studying the links between the growth of SMBHs and their host galaxies has also required progress in the techniques to measure galaxy physical properties in the presence of an AGN.

AH12 presented some of the early results from Herschel, which launched in 2009 (Pilbratt et al., 2010) and provided unprecedented sensitivity to far-IR wavelengths (55–671µm). These wavelengths probe the black-body emission associated with dust that is heated through star-formation process and as such can provide a robust tracer of the total SFR of galaxies that is not impacted by dust obscuration (e.g. Kennicutt, 1998; Lutz, 2014). The emission from the hotter dust heated by an AGN peaks instead at mid-IR wavelengths (see §4.1.3) and is relatively weak at far-IR wavelengths. As such, over the last decade Herschel has been vital as a means of verifying other AGN selection approaches, when the lack of strong far-IR emission can exclude a star-formation origin and confirm that a galaxy is an AGN (e.g. Del Moro et al., 2013; Andonie et al., 2022).
Herschel has also provided “clean” measurements of SFRs for moderate-to-high luminosity AGN in cases when the AGN light dominates at other wavelengths (e.g. Rosario et al., 2012; Stanley et al., 2015). The possible contribution of cooler, extended but AGN-heated dust at far-IR wavelengths remains a topic of debate (e.g. Symeonidis et al., 2016; Lyu and Rieke, 2017; Xu et al., 2020; Symeonidis, 2022); at the highest AGN luminosities the AGN light may again dominate and prevent accurate measurements of the SFR (e.g. Symeonidis and Page, 2018). Herschel’s sensitivity limits also means that it is only able to detect the highest SFR galaxies at a given redshift, providing biased samples of galaxies and an incomplete sampling of AGN hosts.

Measuring the SEDs of galaxies—the intensity of the observed light over a wide range in wavelengths—and fitting to models of the underlying stellar populations thus remains the primary tool to determine SFRs, stellar masses, and other galaxy properties (see Iyer et al., 2025, for a pedagogic overview of the SED-fitting process). In studies of galaxy evolution, recent advances have included improvements to stellar population models (e.g. Stanway and Eldridge, 2018; Conroy et al., 2018; Götberg et al., 2018), more flexible methods to provide accurate descriptions of the diversity/burstiness of star formation histories (e.g. Leja et al., 2019; Robotham et al., 2020), and the use of spectroscopic data alongside photometry (e.g. Chevallard and Charlot, 2016; Carnall et al., 2018). However, the main challenge when studying the host galaxies of AGN (identified using *any* of the methods described in §4.1.1–4.1.4) is to separate the light produced by the galaxy from that produced by the AGN. To this end, many SED fitting codes now allow an AGN component to be included at UV/optical/IR wavelengths (e.g. Calistro Rivera et al., 2016; Boquien et al., 2019; Rosario, 2019; Buchner et al., 2024).<fnref n="9" /> More recent advances have included extending SED fitting into the X-ray and radio regimes, which can provide further constraints on the AGN contribution and thus improve constraints on the underlying host galaxy properties (e.g. Yang et al., 2020, 2022a; Thorne et al., 2023; Azadi et al., 2023). High-resolution optical and near-IR imaging can also enable the central AGN light to be spatially distinguished from the wider host galaxy, improving constraints on both galaxy and AGN properties. Such approaches are now being applied to JWST imaging as well as high-quality ground-based datasets from new facilities such as HyperSuprimeCam (HSC) on Subaru, allowing host-galaxy measurements to be extended to bright unobscured quasars where the AGN dominates the integrated SED (e.g. Li et al., 2021; Ding et al., 2023).

While only able to carry out relatively targetted studies due to the limited field-of-view, ALMA provides exquisite angular resolution and is uniquely capable of measuring certain galaxy properties, such as the gas content via either the sub-mm continuum emission or through molecular emission lines (Combes, 2018). ALMA is also able to constrain the gas dynamics, and thus track in detail the inflow or outflow of molecular gas, as well as measuring dynamical masses — one of the few acces-

sible probes of host galaxy mass when considering the most luminous/highest redshift AGN (see §5.4.1).

These new facilities and approaches have vastly improved our ability to measure host-galaxy properties, even when a bright AGN is present. However, we cannot completely mitigate the impact of the host-galaxy emission on the various AGN selection strategies described in §4.1.1-§4.1.4. New conceptual approaches have thus also been a key driver of progress over the last decade that we explore next.

## 4.2. The varying growth of black holes alongside the galaxy population

Over the last decade, our understanding of the links between galaxies and the growth of their SMBHs has undergone a conceptual advance: recognizing AGN as events within the lifecycles of galaxies. During such events, which may correspond to relatively short phases (< 100 Myr) compared to the overall lifetime of a galaxy (~Gyr), the SMBH may accrete gas, generate emission at different wavelengths (producing the various signatures used to identify AGN described in §4.1), launch relativistic jets, or drive winds and outflows of gas, all at varying rates.<fnref n="10" /> A given galaxy may experience multiple AGN events throughout its lifetime, and thus experiences multiple such AGN phases, returning in-between to a phase when the central SMBH is relatively inactive.<fnref n="11" /> As such, the rate of SMBH growth can vary substantially as it proceeds alongside the evolution of the galaxy population. In the following, we first discuss how this picture emerged over the last decade from early simulation work and observations, before describing the current state-of-the-art, the related (re-)interpretation of AGN downsizing, the connection to obscuration properties, and the discovery of “echoes” of this varying SMBH growth.

### 4.2.1. Early theoretical and observational developments

Patterns of variable SMBH growth were predicted in early simulation work, which showed that the rate at which gas is driven from kpc scales to the sub-pc scales of the SMBH can vary substantially over galaxy timescales. Initial studies focussed on SMBH fuelling within elliptical galaxies (e.g. Ciotti et al., 2009; Novak et al., 2011) and showed how—even in galaxies that are initially devoid of gas and have passively evolving stellar populations—that gas (primarily produced by stellar winds) may be driven onto the central SMBH leading to intermittent phases of AGN activity over galaxy timescales (see also Yuan et al., 2018; Choi et al., 2024). Variations in the rate at which cold gas is delivered to the galactic centre are also expected, whether purely from stochastic accretion of distinct molecular clouds (e.g. Hopkins and Hernquist, 2006) or variations in the flow rate of gas that is driven into the centres of galaxies by secular processes (see §3) or galaxy mergers (see

<footnote n="9">⁹See Pacifici et al. (2023) for an overview and comparison of contemporary SED-fitting codes.</footnote>

<footnote n="10">¹⁰Variations also occur in AGN on much shorter timescales, including variations in emission properties on ~days to ~years, while substantial changes in the accretion rate are observed on decadal timescales in some changing-look AGN (see §3.5).</footnote>

<footnote n="11">¹¹The potential to still produce powerful jets even when the accretion rate is low—leading to a distinct jet-dominated AGN phase—is discussed in §4.3.</footnote>
§4.5). The fuelling rate will also be modified by localised feedback processes from the resulting AGN that may lead to additional variation in the instantaneous rate of accretion onto the SMBH (e.g. Negri and Volonteri, 2017).

The bulk of early observational studies linking AGN and their host galaxies used either narrow emission line or X-ray selection (for studies in the local and more distant Universe, respectively), which trace the radiatively efficient AGN phase (and thus the bulk of SMBH mass growth through accretion) and leave the host galaxy optical SED relatively uncontaminated (see §4.1). Early studies found that most of the AGN hosts in these samples lie in either the “red sequence” or, primarily, the “green valley” of the galaxy colour-magnitude diagram (e.g. Nandra et al., 2007; Silverman et al., 2008; Hickox et al., 2009; Schawinski et al., 2009, 2007). However, by the time of AH12 it was realised such colours are in fact typical of moderate-to-high stellar mass galaxies ($M_* \gtrsim 10^{10} M_\odot$) that are found to host AGN (e.g. Silverman et al., 2009; Cardamone et al., 2010; Xue et al., 2010). The predominance of high-mass galaxies in AGN samples was initially thought to indicate that a massive host is a strong requirement to drive SMBH growth.

This viewpoint was challenged by Aird et al. (2012) who showed that the incidence of X-ray AGN in galaxies across the full stellar mass range is characterised by a power-law shaped distribution of specific black hole accretion rates (sBHAR, denoted $\lambda_{sBHAR} \propto L_{AGN}/M_*$)<fnref n="12" /> that likely reflects the varying rates of AGN activity on short timescales compared to the evolution of the host galaxy (Aird et al., 2013). The prevalence of high $M_*$ hosts in AGN samples is predominantly a selection bias: AGN in more massive galaxies produce higher observable luminosities for a given $\lambda_{sBHAR}$ making them easier to detect and leading to their dominance in flux-limited AGN samples. Subsequent studies have confirmed that this overall selection bias holds over a broader range in redshift (e.g. Bongiorno et al., 2012; Georgakis et al., 2017; Aird et al., 2018; Yang et al., 2018b; Birchall et al., 2022) and for other AGN selections (e.g. Mendez et al., 2013; Trump et al., 2015; Ji et al., 2022). Thus, substantial central SMBH growth appears to be much more widespread across the galaxy population than was previously thought.

Also shortly after the AH12 review, Mullaney et al. (2012a) used new Herschel surveys to identify reliable samples of star-forming galaxies (independent of AGN content, see §4.1.5) and by stacking the X-ray emission demonstrated the existence of a “hidden AGN main sequence” whereby the average SMBH growth rate increased with stellar mass in an analogous manner to the increase in the SFRs of galaxies with stellar mass. The ratio of average SMBH growth relative to galaxy growth thus

remains roughly constant across the star-forming galaxy population. However, the AGN main sequence is “hidden” by the broad distribution of $\lambda_{sBHAR}$ that reflects the varying instantaneous rates of SMBH growth.<fnref n="13" />

Contemporaneous studies focusing on correlations between AGN luminosity and SFR (i.e. tracers of SMBH growth versus galaxy growth) appeared to show contradictory results. Studies that took samples of AGN and used Herschel data (or other probes) to estimate galaxy-wide SFRs found little or no correlation between the observed AGN luminosity and the SFRs of their hosts, at least at low-to-moderate AGN luminosities, although a rise in the typical SFRs toward higher redshift that tracks the overall evolution of the star-forming main sequence was apparent (e.g. Shao et al., 2010; Mullaney et al., 2012b; Harrison et al., 2012; Santini et al., 2012; Rosario et al., 2012; Stanley et al., 2015). In contrast, studies that selected samples of bright, star-forming galaxies detected by Herschel and averaged the amount of SMBH growth through X-ray stacking analysis (following Mullaney et al., 2012a) found a strong correlation whereby the average AGN luminosity increases with SFR (e.g Symeonidis et al., 2011; Chen et al., 2013).

### 4.2.2. Variability on mega-year timescales explains the AGN-star formation connection

Hickox et al. (2014), building on an idea proposed in the AH12 review, demonstrated how the results discussed above could be reconciled by considering the variability of AGN activity over galaxy timescales. They proposed that the galaxy-wide SFR is correlated with the average SMBH accretion rate but the instantaneous SMBH accretion is drawn from a very broad distribution that reflects variability on ~Myr timescales.

Fig. 18 summarizes this conceptual picture. At a given redshift, galaxies can be divided into two populations (left panels): star-forming galaxies, which mostly follow a reasonably tight correlation between total stellar mass, $M_*$, and SFR (the “galaxy main sequence of star formation”, see e.g. Noeske et al., 2007; Elbaz et al., 2011; Popesso et al., 2023); and quiescent galaxies where star formation has quenched and SFRs are much lower, which dominate the galaxy number densities at the highest $M_*$ (at $z \lesssim 1$). Most radiatively efficient AGN <fnref n="14" /> over the bulk of cosmic time (i.e. at $z \lesssim 3$) are found within main-sequence star-forming galaxies (e.g. Rosario et al., 2013; Aird

<footnote n="12">¹²Specific black hole accretion rates are often converted to Eddington ratio equivalent units, given by $\lambda_{sBHAR} = L_{AGN}/(1.3 \times 10^{38} \times 0.002 \times M_*/M_\odot)$ where $L_{AGN}$ is the bolometric AGN luminosity in erg s⁻¹ (e.g. Aird et al., 2012, 2018). While this choice is practical and accounts for an overall scaling with mass, a direct equivalence with Eddington ratios, $\lambda_{Edd}$ (see §3.4), is only true if a universal scaling between $M_{BH}$ and $M_*$ holds at all redshifts; recent observations indicate SMBH masses can span > 2 orders of magnitude at a given $M_*$ at $z \sim 0$ (see Fig. 15). A certain radiative efficiency must also be assumed (typically ≈ 0.1) to convert $\lambda_{sBHAR}$ into a tracer of SMBH mass growth; however, this quantity may also vary considerably between different AGN phases.</footnote>

<footnote n="13">¹³On a philosophical aside, it is interesting to note that while the studies of Aird et al. (2012) and Mullaney et al. (2012a) did take advantage of new facilities and new data, it was the limitations of the available galaxy samples that contained only a subset of known X-ray AGN that motivated the need for new analysis techniques (measuring the probability distribution of accretion rates or stacking to probe average accretion rates in each case) and ultimately led to the conceptual advance to interpret these results in terms of varying growth of SMBHs over galaxy timescales. New data is helpful, but sometimes limitations in our data push us to think in new ways!</footnote>

<footnote n="14">¹⁴We refer here to radiatively efficient AGN as those where a standard accretion disk is present that ensures an efficient production of radiation (primarily at UV/optical wavelengths but also producing the X-ray and IR emisson, as discussed in §4.1); see Fig. 11. Such an accretion mode is assumed to be operational when the SMBH is accreting at Eddington ratios $\gtrsim 1\%$, or the corresponding $\lambda_{sBHAR}$. However, radiatively efficient AGN may still be identified with $\lambda_{sBHAR} < 0.01$, in part due to the large scatter in the scaling between $M_{BH}$ and $M_*$ (see Footnote 12).</footnote>
<figure><img src="image_19.png" /></figure>

Figure 18: Diagram illustrating the picture established over the last decade that explains the relation between AGN and galaxy SFRs over the bulk of cosmic time (based on results at $z \lesssim 3$). Star-forming galaxies mostly lie on the “main sequence of star formation” such that (at a given redshift) higher stellar mass galaxies have higher SFRs (left panel). The SFR of a galaxy varies relatively slowly on ~Gyr scales, leading to a relatively narrow distribution of specific SFRs (central upper panels). AGN may be viewed as multiple, relatively short events (lasting $\lesssim 100$ Myr) within the lifetime of a galaxy when the rate of accretion onto the central SMBH exceeds a certain level (highlighted in green in the lower central panels). The accretion rate may also vary substantially on shorter timescales during AGN and inactive phases, leading to a broad distribution of accretion rates. Strong jets can still be produced even during phases when the accretion rate is low (highlighted in orange) and other AGN signatures are absent, corresponding to a “jet-dominated” or “radiatively inefficient” AGN phase (see Figs. 22 and 23 and discussion in §4.3). The broad distribution of accretion rates, due to this short-timescale variability, blurs any direct correlation between the total (radiative) AGN luminosity and the SFR (green and orange dots in right panel). A similar average SFR is found at different AGN luminosities, corresponding to main sequence of moderate-mass galaxies at a given redshift (black squares). In contrast, the average AGN luminosity for galaxies of a given SFR reveals a correlation (blue circles) showing that—once short-term variations in AGN luminosity are averaged out—higher SFRs are associated with increased growth rates of central SMBHs.

et al., 2018). The distributions of specific SFRs (sSFRs) for these galaxies will be comparatively narrow (top central panels of Fig. 18, e.g. Mullaney et al., 2012a; Schreiber et al., 2015). In contrast, the instantaneous rate of accretion onto the central SMBH is highly variable and can span a wide range (bottom central panels of Fig. 18). Over galaxy evolutionary timescales (~Gyr) numerous AGN events (typically lasting <100 Myr) may occur when the SMBH switches between inactive phases when the rate of mass accretion is relatively low (highlighted in orange in Fig. 18), and AGN-dominated phases where the SMBH is accreting at a high rate (highlighted in green). Thus, within a sample of star-forming galaxies with a relatively narrow range of sSFRs, a broad distribution of sBHARs is found, blurring any direct correlation between SFR and AGN luminosity (right panel of Fig. 18). Selecting galaxies of a given SFR and averaging their AGN luminosity (e.g. through X-ray stacking), thus averaging over the variable sBHAR distribution, reveals the underlying correlation (blue circles in right panel of Fig. 18). In contrast, AGN selected at different luminosities all have a wide range of SFRs with an average consistent with the main sequence of star formation at the corresponding redshift but no correlation is found (except possibly at the highest AGN luminosities, which require high mass host galaxies that have higher SFRs, producing a weak correlation, e.g. Rosario et al., 2012; Dai et al., 2018b).

### 4.2.3. Our current picture of SMBH growth as a function of stellar mass, SFR and galaxy structure

A result of this conceptual shift is the realisation that studying the properties of an individual AGN is insufficient to determine how host properties affect SMBH growth as the instan-

taneous AGN luminosity is a poor indicator of the overall accretion rate on cosmic timescales. Instead, new observational techniques have been developed to quantify the level of AGN activity in terms of both the occurrence of AGN events and the overall SMBH mass growth in different galaxy populations, primarily through measurements of the probability distribution of $\lambda_{sBHAR}^{15}$ and derived quantities such as the fraction of galaxies with an AGN (above specified limits in luminosity or $\lambda_{sBHAR}$) or sample-averaged accretion rates for different galaxy populations (e.g. Bongiorno et al., 2016; Yang et al., 2017a; Georgakakis et al., 2017; Yang et al., 2018b; Zou et al., 2024).

Fig. 19 summarises the emerging picture of how such measurements depend on a number of key galaxy properties. In general, $p(\lambda_{sBHAR})$ has a broad shape with a break at high $\lambda_{sBHAR}$ corresponding to ~the Eddington limit and thus reflecting the (likely) self-regulation of SMBH growth through localised feedback mechanisms (e.g. Aird et al., 2013; Bongiorno et al., 2016; Georgakakis et al., 2017; Zou et al., 2024). AGN are found throughout the star-forming main sequence, but $p(\lambda_{sBHAR})$ shifts toward higher values at higher $M_*$, producing both a slight increase in AGN fraction (indicating a higher rate of triggering of AGN events) and higher average accretion rates (e.g. Aird et al., 2018; Yang et al., 2018b). The total galaxy stellar mass may be the primary determinant of the absolute level of SMBH mass growth in such galaxies (e.g. Yang et al., 2017a). However, Aird et al. (2019) suggest that—once an overall scaling with stellar mass is accounted for via measure-

<footnote n="15">¹⁵These probability distributions, that we refer to as $p(\lambda_{sBHAR})$, are defined as the probability density of an AGN of a given $\lambda_{sBHAR}$ occurring in a sample of galaxies of a certain type/range of properties (e.g. stellar mass).</footnote>
ments of $\lambda_{\text{SBHAR}}$—the absolute SFR provides a better predictor of AGN activity along the main sequence while also accounting for the strong increase in AGN fraction toward higher redshift i.e. from $z \sim 0$ to $z \sim 3$, where the normalisation of the main sequence and thus typical SFRs for galaxies of given $M_*$ also increase (see also Mountrichas et al., 2022b). Such scalings with SFR suggest that it is the overall availability of cold gas—whether stochastically delivered to the central regions of a galaxy or indirectly as the fuel to form stars that drive strong stellar winds—that determines both the rate of AGN events and the overall amount of SMBH mass growth that takes place in a galaxy on cosmic timescales. However, determining whether stellar mass or SFR is the primary determinant of AGN activity remains difficult for main-sequence star-forming galaxies where these quantities are correlated.

An additional determining factor in SMBH growth may be the prominence of a central stellar bulge. Indeed, higher mass star-forming galaxies tend to be more bulge-dominated and thus bulge mass, rather than total stellar mass, may be more closely related to AGN activity (e.g. Yang et al., 2019a). Considering instead galaxies with different SFRs relative to the main sequence (at a given $M_*$), the subset of galaxies with slightly lower SFRs (“sub-main-sequence” galaxies) show a higher incidence of AGN (e.g. Shimizu et al., 2015; Aird et al., 2019; Ji et al., 2022) that may correspond to their typically more centrally concentrated stellar densities (i.e. more compact sizes) which also appears to be an important determinant of AGN fraction and sample-averaged accretion rates (e.g. Rangel et al., 2014; Kocevski et al., 2017; Ni et al., 2019, 2021b; Aird et al., 2022). However, it is important to note that a central stellar bulge is not a requirement for a galaxy to grow a central SMBH or host an AGN (e.g. Simmons et al., 2013). Starburst galaxies, which have enhanced SFRs relative to their stellar masses and thus lie above the main sequence, also exhibit an enhancement in AGN fraction (e.g Bernhard et al., 2016; Aird et al., 2019).<fnref n="16" /> Gas-rich major mergers may be an important route to trigger this important sub-set of the AGN population (see §4.5 and §5.3 for further discussion).

Finally, we note that AGN are fairly widespread within the quiescent galaxy population and while they typically have lower $\lambda_{\text{SBHAR}}$ there may still be substantial SMBH growth occurring in such galaxies (e.g. Wang et al., 2017; Aird et al., 2018; Birchall et al., 2023). These galaxies, by definition, have little ongoing star formation and lack a ready supply of cold gas that is thought to be the main driver of SMBH growth in star-forming galaxies. Several studies have found that the AGN incidence in such galaxies anti-correlates with direct tracers of stellar age (determined from spectroscopic tracers or through modeling of stellar populations, e.g. Kauffmann and Heckman, 2009; Hernán-Caballero et al., 2014; Mountrichas et al., 2022a; Georgantopoulos et al., 2023; Ni et al., 2023; Almaini et al., 2025) or tracks another property that is known to correlate with age (e.g. size,<fnref n="17" /> see Rangel et al. 2014; Aird et al. 2022).

Such findings suggest that recycled gas—produced by the mass loss through supernovae and stellar winds as a stellar population evolves—may be a crucial source of fuel for AGN in such galaxies, as predicted in theoretical works discussed above (e.g. Ciotti et al., 2009; Choi et al., 2024). In the most massive quiescent galaxies, the hot gas reservoir may be sufficient to fuel frequent AGN events at relatively low accretion rates, either directly (e.g. Yuan and Narayan, 2014) or via a fraction that condenses into cold gas and is accreted stochastically by the SMBH (e.g. Gaspari et al., 2015), further enhancing the AGN incidence at low sBHARs in such galaxies (e.g. Ni et al., 2023).

### 4.2.4. Re-assessing AGN downsizing

The discovery of the broad distribution of specific SMBH accretion rates, and the associated development of our understanding of how host galaxy properties affect SMBH growth, also feeds into our interpretation of the space density of AGN of different luminosities (i.e. the AGN luminosity function) and the observed “downsizing” pattern with redshift (see §4.1.2, Fig. 17). The total space density of AGN at a given luminosity is the result of a convolution of the space density of galaxies of a given stellar mass (i.e. their stellar mass function, SMF) and the broad distribution of $\lambda_{\text{SBHAR}}$ (see Fig. 20). Due to the relative steepness of $p(\lambda_{\text{SBHAR}})$ compared to the SMF, the space density of AGN over a wide range in luminosities ($L_{\text{AGN}} \approx 10^{43} - 10^{46}$ erg s⁻¹) is dominated by host galaxies with masses of $M_* \sim 10^{10} - 10^{11} M_{\odot}$ (lower panel of Fig. 20; Aird et al., 2013; Caplar et al., 2015; Georgakakis et al., 2020), without requiring any preferential mass range for AGN activity. Higher mass galaxies will produce higher luminosity AGN (for the same distribution of $\lambda_{\text{SBHAR}}$), but as such galaxies are much rarer they only make a significant contribution to the total space density of AGN at the highest luminosities. Lower mass galaxies, on the other hand, are more common but produce lower luminosity AGN events. To produce a given AGN luminosity for a lower mass galaxy requires a higher $\lambda_{\text{SBHAR}}$, and such phases are rarer than at lower $\lambda_{\text{SBHAR}}$ (see §4.6).

The interplay between the evolution of the galaxy SMF (in particular the increase in normalisation due to the build up of the galaxy population between $z \sim 4$ and $z \sim 1$) and the evolution of $p(\lambda_{\text{SBHAR}})$ (which shifts toward lower values between $z \sim 2$ and $z \sim 0$, likely due to the decrease in the availability of cold gas) is able to produce the observed “downsizing” in the evolution of the space densities of AGN of different luminosities, as shown in Fig. 17 (Caplar et al., 2015). Differential growth of SMBHs of different masses is not required to produce this pattern. However, independent observations—in particular for bright sub-samples where SMBH masses may be measured directly (see §5.1)—indicate a true downsizing whereby more massive SMBHs appear to be more active at earlier cosmic times than their lower mass counterparts (e.g. Heckman et al., 2004; Kelly and Shen, 2013, see discussion in AH12

<footnote n="16">¹⁶The enhancement of AGN activity in starburst galaxies, however, may not be as strong as the enhancement in the SFRs, see e.g. Rodighiero et al. (2015).</footnote>

<footnote n="17">¹⁷Physically larger quiescent galaxies are found to have younger stellar populations (e.g. van der Wel et al., 2009; Hamadouche et al., 2022), resulting in enhanced AGN fractions in larger / more extended quiescent galaxies (cf. star-forming galaxies where AGN activity appears to be boosted in more compact galaxies).</footnote>
<figure><img src="image_20.png" /></figure>

Figure 19: Schematic showing how the incidence of AGN, quantified by the probability distribution of specific SMBH accretion rates, $p(\lambda_{\text{SBHAR}})$, changes depending on the SFRs and stellar masses (indicated by the axes of the main panel) as well as the structure of galaxies (illustrated by the cartoon galaxies). Galaxies further along the main sequence (i.e. more massive/higher SFR/physical larger star-forming galaxies) are associated with a shift of $p(\lambda_{\text{SBHAR}})$ toward higher $\lambda_{\text{SBHAR}}$ that is likely related to increases in the availability of cold gas (top panels). An enhancement in the fraction of galaxies that host AGN is also found in sub-main-sequence galaxies (moving relative to the main sequence at fixed $M_*$, right panels) that appears to be related to their more compact structure (i.e., smaller physical sizes, as illustrated). AGN are also found in galaxies with lower SFRs (i.e., in quiescent, bulge-dominated galaxies) but typically with lower $\lambda_{\text{SBHAR}}$, likely fuelled by stellar mass loss from a passively evolving stellar population and there may be an anti-correlation with size; see Footnote 17. Radiatively inefficient AGN with low $\lambda_{\text{SBHAR}}$ but significant radio jets are also common in such galaxies (see Fig. 23 & §4.3).
<figure><img src="image_21.png" /></figure>

Figure 20: Illustration of how the AGN luminosity function (space density of AGN as a function of luminosity) at moderate redshifts ($z \lesssim 3$) is built up from the galaxy stellar mass function (space density of galaxies with different stellar masses, top left panel) and the broad probability distribution of specific SMBH accretion rates, $p(\lambda_{\text{SBHAR}})$ (top right panel). Due to the broad range in $\lambda_{\text{SBHAR}}$, galaxies of a given stellar mass (indicated by colours/linestyles in the top left panel) produce AGN with a wide range of luminosities (corresponding colours/linestyles in bottom panel). However, the steepness of $p(\lambda_{\text{SBHAR}})$ relative to the stellar mass function means that host galaxies with $M_* \sim 10^{10} - 10^{11} M_\odot$ (solid green line) dominate the total AGN luminosity function except at the very highest and very lowest luminosities.

§3.3.1). Some of this discrepancy may be due to the broad range of SMBH masses for galaxies of similar stellar mass, allowing for SMBH-mass-dependent growth to occur without a strong dependence on host stellar mass. Indeed, recent observations suggest that the scaling between SMBH and stellar mass depends on host galaxy SFR and has a large scatter (e.g. Terrazas et al., 2017; Greene et al., 2020, see also Fig. 15) that may require a range of SMBH growth pathways even in galaxies of similar stellar mass (e.g. Guetzoyan et al., 2024; Terrazas et al., 2024, see also §5.4).

To fully track the growth of the masses of central SMBHs over cosmic time and reconcile with the assembly of their host galaxies requires detailed modelling either through data-driven techniques that combine multiple observation constraints (e.g. Shankar et al., 2013; Georgakakis et al., 2020; Zhang et al., 2023) or from physically-motivated hydrodynamical simulations (e.g. Habouzit et al., 2021). However, a consensus view is yet to emerge.

### 4.2.5. Obscuration properties and their connection to varying SMBH growth rates

A remaining open question is how the obscuration properties of AGN fit into this scheme of distinct AGN events with varying levels of SMBH growth throughout a galaxy lifetime. Line-of-sight obscuration may occur due to gas within the wider ISM of the host galaxy (e.g. Buchner et al., 2017; Gilli et al., 2022) and as such be associated with the same reservoir of gas responsible for fuelling AGN events. As such, the level of obscuration may differ between different galaxy types due to the distinct physical conditions and AGN triggering mechanisms at play across the galaxy population (e.g. Rangel et al., 2014; Georgantopoulos et al., 2023; Mountrichas et al., 2024). The parsec-scale material associated with the “torus” will also be a major contributor, with the line-of-sight column density dependent on viewing angle (Netzer, 2015; Buchner and Bauer, 2017). Winds, driven by the central accretion disk, can also introduce variations in line-of-sight obscuration that depend on the viewing angle. Distinct phases that form a sequence within a single AGN event, as gas is funnelled into the central regions, fuels accretion onto the central SMBH and the associated AGN, and is subsequently expelled from the central regions due to feedback from the AGN itself, can also lead to variations in the level of obscuration (see §5.3). Indeed, the obscuration may be due to the same material that is ultimately accreted by the SMBH (see §3) and is eventually disrupted by localised feedback processes (e.g. radiation pressure, see Ricci et al., 2017c; Laloux et al., 2024) and as such closely linked to a cycle of fuelling and feedback that determines the length of AGN events.

### 4.2.6. Echoes of past AGN activity

Given the emerging view of AGN as events in galaxies, we might expect to find evidence of past AGN in galaxies that are currently in an inactive phase. Indeed, a new sub-field has emerged over the past decade that involves observational searches for such evidence (e.g. Keel et al., 2017; Lansbury et al., 2018). One of the first and clearest indicators of such past activity was discovered thanks to the engagement of a broader community of citizen scientists through the Galaxy Zoo project (Lintott et al., 2008, 2011), which provided images of galaxies from the SDSS to the general public to classify based on their visual morphologies, a task that was insurmountable for professional astronomers alone. This process also enabled the identification of rare and unusual sources. One such finding was “Hanny’s Voorwerp”,<fnref n="18" /> a highly irregularly shaped object that appears extremely green in the SDSS images. This object has subsequently been attributed to an ionisation “echo” related to past AGN activity in the centre of the nearby galaxy, IC 2497 (Lintott et al., 2009) analagous to the extended emission line regions of known AGN (e.g. Stockton et al., 2006; Keel et al., 2022) but lacking the central power source. The discovery has prompted wider searches for similar ionisation

<footnote n="18">¹⁸Discovered by Dutch schoolteacher Hanny van Arkel, a citizen scientist participant in Galaxy Zoo. The name “Voorwerp” is Dutch for object or “thingy” but has now been adopted by the professional astronomy community as the generic term for these objects associated with AGN light echoes.</footnote>
echos, leading to the discovery of a number of “Voorwerpje” with similar properties (e.g. Keel et al., 2012). Schawinski et al. (2015) used the incidence of such Voorwerpje to estimate the length of AGN events and proposed an evolutionary sequence (reproduced in Fig. 21) linking the start of an AGN event, the production of extended ionisation cones during the AGN phase, and the remnant light echoes seen as the Voorwerpje when the central SMBH drops in accretion rate and returns to an inactive phase. Evidence of the impact of past AGN jets (see §4.1.4 and 4.3) is also seen in remnant radio sources across a range of systems (see Mahatma, 2023, and references therein), including in Hanny’s Voorwerp (Smith et al., 2022).

There is also evidence that the SMBH at the centre of the Milky Way, Sagittarius A*, has undergone multiple past AGN phases, including the presence of bubbles of hot gas seen at Gamma-ray wavelengths (with Fermi, Su et al., 2010) and at X-ray wavelengths (most clearly with new all-sky data from eROSITA, Predehl et al., 2020) that extend several kiloparsecs above and below the Galactic plane, although the possibility of a non-AGN origin is still under debate (e.g. Yang et al., 2022b; Liu et al., 2024; Churazov et al., 2024; Sarkar, 2024).

### 4.3. Jetted AGN – capping the growth of galaxies and their black holes

The discussion in §4.2 above has focussed on AGN that are identified by their radiative output: most effectively identified at optical/UV, X-ray, or IR wavelengths. However, the last decade has seen a drastic increase in our observational capabilities at radio wavelengths that enable us to trace another form in which SMBHs send energy out into the wider environment: highly collimated, powerful jets. While most clearly observed at radio wavelengths (see §4.1.4), the bulk of the power in such jets is carried by the kinetic energy of the outflowing material rather than emerging as electromagnetic radiation. In this section, we discuss how the presence of jets relates to the broader AGN population, the different phases of activity, and our understanding of AGN as “events”. We also discuss recent studies of how such jetted AGN populate galaxies and highlight the role that jetted AGN are thought to play in capping the ultimate growth of the stellar mass of their host galaxies and the mass growth of SMBHs themselves.

First, it is important to note that strong, extended jets are associated with at least some fraction of radiatively efficient AGN that also possess a standard accretion disk and the many other features of this phase of activity (e.g. Whittam et al., 2018; Kondapally et al., 2025). Whether a jet is always present in a radiatively efficient AGN—but may be weaker/less extended in some cases—is still unclear, in part due to the difficulty in distinguishing such jets from other radio-emitting processes (e.g. stellar processes, AGN driven-winds; see discussion in §4.1.4).

Jets are also produced when the SMBH is in a radiatively inefficient accretion mode (i.e. a RIAF/ADAF, see §3.4 and Fig. 11), typically corresponding to $\lambda_{Edd} \lesssim 1\%$, or the equivalent $\lambda_{sBHAR}$, and when the mass accretion rate is usually relatively low. This dichotomy is reflected in the observational classification of radio-selected AGN into high-excitation radio

galaxies (HERGs) and low-excitation radio galaxies (LERGs)<fnref n="19" /> that broadly correspond to radiatively efficient and inefficient regimes, respectively. Fig. 22 illustrates these different phases of activity of a SMBH (as well as completely inactive phases), where the green and yellow highlighting is chosen to connect to the phases of high and low radiative output (in terms of $\lambda_{sBHAR} \approx \lambda_{Edd}$) as in Figs. 18–20.<fnref n="20" /> We stress that the different phases of SMBH activity illustrated in Fig. 22—including additional, intermediate phases where the various emission components range in strength—may correspond to discrete AGN “events”, and thus can occur in distinct host galaxies with differing duty cycles and distinct triggering/fuelling mechanisms. Whether an evolutionary sequence between some of these phases within a single AGN event occurs, at least in some cases, remains an important open question.

In the local ($z \lesssim 0.3$) universe, a strong rise in the fraction of galaxies with a radio-emitting AGN (predominantly radiatively inefficient, jet-dominated sources) with increasing stellar mass is well established (see AH12 §3.2.1; Best et al., 2005; Heckman and Best, 2014). Recent large-area, sensitive surveys with LOFAR confirm this pattern and indicate that at high stellar masses ($M_* > 10^{11} M_\odot$) a radio-emitting, albeit low-luminosity jet is always present and revealed in sufficiently deep imaging (e.g. Sabater et al., 2019, see Fig. 23, bottom panel). Low-frequency radio emission may persist up to timescales $\sim 10^8$ yr, and thus some of the radio detections likely correspond to remnants of recent activity rather than indicating the current activity of the central SMBH (cf. the echoes of past radiative AGN accretion discussed in §4.2.6). Indeed, work to identify not only remnant radio sources but also concurrent evidence of re-starting radio jets is emerging as a major field given the sensitivity, frequency range and spatial resolution of the latest radio surveys, enabling detailed insights into the lifecycles of massive galaxies and their central SMBHs (e.g. Shabala et al. 2020; Jurlin et al. 2020, see Morganti 2024 for a detailed recent review). The high radio AGN fractions in low-redshift massive galaxies require repeated and near-continual fuelling of the central SMBH, albeit at low levels. Such galaxies are predominantly quiescent in terms of star formation and generally lack a ready supply of cold gas but contain large reservoirs of hot gas. Gradual cooling of this hot gas is likely sufficient to provide the low-but-continual accretion rates needed to sustain the high

<footnote n="19">¹⁹The original classification of radio galaxies as HERGs or LERGs was based on the presence or absence, respectively, of high-excitation emission lines in their optical spectra (e.g. Best and Heckman, 2012) but as deeper radio surveys have become available the definitions have expanded to refer more generally to sources where the radio emission is dominated by the AGN and a radiatively efficient AGN is identified in the broader multiwavelength SED (i.e. the detection of an X-ray AGN or an optical-to-IR SED that indicates the presence of a quasar or obscured AGN) in the case of a HERG, or that these other signatures of a radiative AGN are absent and only the radio emission indicates the presence of an AGN in the case of a LERG (e.g. Best et al., 2023; Kondapally et al., 2025).</footnote>

<footnote n="20">²⁰We note that discrepancies between $\lambda_{Edd}$ and $\lambda_{sBHAR}$, given the range of $M_{BH}$ in galaxies of a given $M_*$ (see Footnote 12 and Fig. 15) result in an imperfect correspondence between radiatively efficient ($\lambda_{Edd} > 0.01$) and inefficient/jet-dominated ($\lambda_{Edd} < 0.01$) AGN phases and different $\lambda_{sBHAR}$ values. It is also important to note that the SMBH mass accretion rate may still be relatively high in low-$\lambda_{Edd}$ but radiatively inefficient phases.</footnote>
<figure><img src="image_22.png" /></figure>

Figure 21: Schematic illustration of how an AGN event occurring in a galaxy leads to the production of extended emission line regions (the ionization cone) during the AGN phase and subsequently leaves a short-lived “echo” of prior activity when the SMBH returns to inactive phase, revealed via the discovery of Voorwerpe around a fraction of nearby galaxies. Source: reproduced from Fig. 1 of Schawinski et al. (2015).

<figure><img src="image_23.png" /></figure>

Figure 22: Illustration of different phases of a central SMBH (cf. Fig. 1 which shows size scales), including radiatively-efficient phases (left two diagrams, highlighted in green) corresponding to $\lambda_{Edd} \gtrsim 1\%$ when a standard accretion disk (shown in blue) and broad-line region (purple), narrow-line region (green), X-ray corona (cyan) and warm dusty obscurer (red, with extended outflows of hotter dust in the polar direction shown in brown as in Fig. 10) are all present and a radio jet (orange) may be present but may be weak/small (first panel) or strong and extend up to ~Mpc scales (second panel). At lower accretion rates (corresponding to $\lambda_{Edd} \lesssim 1\%$, right two diagrams highlighted in yellow) a strong radio jet may still be produced, corresponding to a radiatively inefficient AGN phases when the accretion is in the form of a RIAF/ADAF (shown in yellow in third panel, see §3.4 and Fig. 11) or the SMBH may be inactive and neither growing in mass nor powering substantial radiation (i.e. not an AGN, fourth panel). We note that these phases all likely form a continuum; they are *not* four completely distinct phases. Whether such phases can occur in a sequence as part of a single AGN “event”—and if so, their order, how long each phase lasts, and the persistence of different structures across phases—or whether they only occur as part of distinct events remains a key open question (see also §5.3 for discussion of additional phases that may occur as part of an evolutionary sequence for the brightest quasars).
<figure><img src="image_24.png" /><figcaption>Figure 23: Top: Cumulative fraction of radio-selected AGN with specific Black Hole Kinetic Power ($\lambda_{sBHKP} \propto P_{kin}/M_*$) exceeds a given value, providing a tracer of the relative power carried by the jet. We have divided the radio-selected AGN into those with high radiative power ($\lambda_{sBHAR} > 0.01$) and low radiative power ($\lambda_{sBHAR} < 0.01$), which should roughly correspond to radiatively efficient and inefficient AGN phases, respectively. Both AGN phases are associated with a broad range of jet power, although the strongest jets are mostly associated with radiatively efficient AGN. Source: produced using results from Kondapally et al. (2025). Bottom: Fraction of galaxies with a radio AGN above different radio luminosity thresholds, showing that the most massive galaxies always produce some level of radio emission and thus indicating that their SMBHs are “always switched on” i.e. producing radio jets. Source: reproduced from Fig. 5 of Sabater et al. (2019).</figcaption></figure>

duty cycle of jetted AGN. The cooling hot gas may condense along filaments and could thus in fact fuel the SMBH through a stochastic supply of cold gas (e.g. Gaspari et al., 2015) that triggers the initial AGN event and leads to brief, radiatively efficient AGN phases (see §4.2.3) before the SMBH enters a more sustained, radiatively-inefficient jet-dominated phase.

The substantial increase in the incidence of jetted AGN in galaxies with stellar mass persists even if an overall scaling of jet power with stellar mass is accounted for (e.g. Igo et al. 2024, although see also Hardcastle et al. 2019 for discussion of the impact of differing source lifetimes across the mass scale). The total kinetic power carried by jets, $P_{kin}$, may be estimated from the cavities they appear to inflate in the hot IGM of massive clusters or via assumptions about the synchrotron emitting plasma, as these are found to be in balance (Bîrzan et al., 2004;

Rafferty et al., 2006; Timmerman et al., 2022). $P_{kin}$ is found to scale (albeit with large scatter) with the jet radio luminosity<fnref n="21" /> and extrapolating this relationship allows estimates of the jet power more generally across the radio AGN population. We consider here the specific black hole kinetic power, sBHKP, $\lambda_{sBHKP} \propto P_{kin}/M_*$ where $M_*$ is the host galaxy stellar mass, as a mass-normalised tracer of the jet power in a manner analogous to the adoption of $\lambda_{sBHAR}$ that provides a normalised measure of radiative power <fnref n="22" /> (e.g. Igo et al., 2024). The top panel of Fig. 23 shows the distribution of $\lambda_{sBHKP}$ for radio-selected AGN (i.e. sources where the radio emission is predominantly due to AGN) that are identified in deep LOFAR surveys (Kondapally et al., 2025), divided into those with higher radiative power ($\lambda_{sBHAR} > 0.01$, where the radiative AGN luminosity, $L_{AGN}$, has been estimated via SED fitting) and lower radiative power ($\lambda_{sBHAR} < 0.01$), which should broadly correspond to radiatively efficient and inefficient phases.<fnref n="23" /> It is clear that jets spanning a broad range of $\lambda_{sBHKP}$ are found across both AGN phases and weaker jets dominate in both cases. However, the highest jet power sources are associated with high radiative powers (i.e. high $\lambda_{sBHAR}$). Thus, the power carried by jets appears to have a weak—but still significant—relationship to the current growth rate of the central SMBH.

Over the last decade, new facilities have allowed radio surveys to reach sufficient sensitivity to probe the demographics of the radio source population outside the local universe, reaching to $z \sim 4$ (e.g. Delvecchio et al., 2017, 2018; Kondapally et al., 2022; Whittam et al., 2024). The overall evolution of the radio luminosity function of the two broad classes of jetted AGN follow somewhat different patterns. Radiatively-efficient, jetted AGN (HERGs) increase in number density toward higher redshift in a similar manner to the broader population of “non-jetted” radiatively-efficient AGN (e.g. Best et al., 2014; Ceraj et al., 2018). In contrast, the number densities of radiatively-inefficient jet-dominated AGN (LERGs) appear to undergo relatively little evolution in space density between $z \sim 2$ and $z \sim 0$ (e.g. Williams et al., 2018; Kondapally et al., 2022). However, this weak overall evolution in space density masks an important transition in their host galaxies from predominantly quiescent in the local universe to predominantly star-forming host galaxies by $z \sim 1^{24}$ (e.g. Whittam et al., 2018, 2022; Kondapally et al., 2022, 2025). Thus, there appears to be a popula-

<footnote n="21">²¹The relation from a compilation of cavity-based measurements by Heckman and Best (2014), giving $P_{kin} = 2.8 \times 10^{37} (L_{14GHz}/10^{25} W Hz^{-1})^{0.68}$ W, is widely adopted. The same quantity may be referred to as $Q$ or as the “mechanical luminosity”, $L_{mech}$.</footnote>

<footnote n="22">²²As with $\lambda_{sBHAR}$ (see Footnote 12), the specific black hole kinetic power can be given in “Eddington-ratio-equivalent” units if a single scaling relation between $M_*$ and $M_{BH}$ is assumed, i.e. $\lambda_{sBHKP} = P_{kin}/(1.3 \times 10^{38} \times 0.002 \times M_*/M_{BH})$ where $P_{kin}$ is the kinetic power in erg s$^{-1}$. Such units provide a useful scale to assess how much kinetic power is being released relative to the theoretical maximum radiative power from accretion onto a SMBH.</footnote>

<footnote n="23">²³As discussed throughout this section, the imperfect correspondence between $\lambda_{sBHAR}$ and $\lambda_{Edd}$ is an important caveat when considering the true division of AGN based on their radiative power. Furthermore, recent studies indicate there is not a simple correspondence between higher and lower $\lambda_{Edd}$ and the classification of sources as either HERGs or LERGs (e.g. Whittam et al., 2022).</footnote>

<footnote n="24">²⁴Kondapally et al. (2025) show that the fraction of quiescent galaxies</footnote>
tion of radiatively-inefficient, jet-dominated AGN that appear
to be fuelled by the same cold gas supply that is thought to trig-
ger radiatively-efficient AGN in star-forming galaxies, lending
support to the idea that both AGN types may in fact be phases
within a *single* AGN event.

The prevalence of jetted AGN in high-$M_*$ galaxies and the fact that (at least by $z \sim 0$) such galaxies tend to have quenched their star formation suggests that jets play a vital role in *cap-ping* the growth of the most massive galaxies – providing a cru-cial feedback mechanism that maintains the large-scale hot gas reservoir in their host halos. By limiting–if not completely halting—the rate of gas cooling and the associated flow of *cold* gas into the central regions, these jets may also cap the mass growth of SMBHs themselves. Our developing knowledge of this delicate feedback cycle is discussed in §6.

4.4. Do AGN preferentially occur in certain large-scale envi-
ronments?

A major driver of the evolution of galaxies and black holes
is their large-scale cosmic environment, which can be param-
eterized by the mass of the host dark matter halo or the prop-
erties of the parent galaxy group, cluster, supercluster, or fil-
ament. Galaxy evolution is connected to environment via a
range of processes including tidal effects, galaxy mergers, gas
inflows and outflows, and interactions with the IGM (e.g., Mo
et al., 2010; Peng et al., 2010). In general, galaxies residing in
denser environments or more massive halos are more likely to
have quenched star formation and older stellar populations, and
there may be a corresponding impact on the fuelling of SMBH
accretion. Different fuelling processes, for example condensa-
tion from a hot atmosphere as opposed to torques from galaxy
mergers, may be more likely to occur in different environments,
making studies of environment a common observational tool for
understanding SMBH growth.

The effects of environment on AGN activity have so far been
less clear than those for galaxy evolution, owing in part to more
limited statistics and the stochastic behaviour of SMBH accre-
tion. Studies of AGN environments have generally followed
one of two approaches: (1) direct studies of the populations
of AGN among member galaxies in groups, clusters, filaments,
and voids, and (2) statistical studies of the spatial correlations of
AGN populations, which enabled inferences as to how the AGN
 populate dark matter haloes. Overall, these studies find signif-
icant connections between AGN activity and environment, but
any *additional* impact beyond the relationships seen for galax-
ies is relatively small for most populations of AGN.

4.4.1. Direct measurements of AGN incidence in groups and clusters

Over the past decade, direct measurements of the AGN pop-
ulations in galaxy groups and clusters have extended to lower
luminosities and higher redshifts thanks to improvements in
observational sensitivity. For X-ray selected AGN, the AGN
fraction in clusters (defined as the fraction of galaxies with
AGN above some $L_X$ threshold) is found to decrease with in-
creasing cluster mass, and decrease with radius toward the cen-
ters of clusters (e.g., Ehlert et al., 2014, 2015; Noordeh et al.,
2020), with cluster outskirts reaching values approximately that
of more typical “field” environments (e.g., Ehlert et al., 2014;
Koulouridis et al., 2024). The X-ray AGN population in clus-
ters also evolves rapidly with redshift, with the AGN fraction
increasing by more than an order of magnitude to $z \sim 1.5$ (e.g.,
Martini et al., 2013), and optical- and IR-selected AGN in clus-
ters show similar behavior (e.g., Alberts et al., 2016; Lopes
et al., 2017). There has been some evidence presented for a
potential excess of AGN outside the cluster virial radius, in-
dicating fuelling of SMBH growth during galaxy infall (e.g.,
Fassbender et al., 2012; Koulouridis and Bartalucci, 2019), but
these effects are not seen in all studies and may simply be ow-
ing to statistical fluctuations and projection effects (e.g., Muñoz
Rodríguez et al., 2024).

Recent years have also seen direct measurements of how
AGN populate galaxy groups. Massive galaxy clusters ($M \gtrsim 10^{14} M_{\odot}$) provide large samples of galaxies and clear identifi-
cations with large-scale structures, but the bulk of the mass
in stars and SMBHs resides in halos of galaxy group scale
($10^{12} \lesssim M \lesssim 10^{13} M_{\odot}$). Increasingly sensitive optical and
X-ray surveys have enabled identification of galaxy groups to
$z \sim 1$ and beyond, with corresponding measurements of how
AGN occupy these systems (e.g., Allevato et al., 2012; Oh
et al., 2014; Gordon et al., 2018). Generally groups are found
to have higher fractions of AGN than clusters, extending the
mass trends observed for clusters, with the incidence similarly
increasing with redshift.

Identification of AGN with groups and clusters has also en-
abled direct measurements of the halo occupation distribution
(HOD), defined as the number of AGN hosted in central and
satellite galaxies as a function of group or cluster mass. These
studies find that the incidence of AGN has a relatively weak
but increasing dependence on halo mass, with a characteristic
$M_{\text{halo}}$ of $\sim 10^{13} M_{\odot}$ and the increasing numbers of AGN at
higher $M_{\text{halo}}$ corresponding to more AGN in satellite galaxies
(e.g. Chatterjee et al., 2013; Chakraborty et al., 2018).

Overall, the trends of AGN fractions increasing with clusto-
centric radius and redshift, and decreasing with cluster group
mass are also observed for star formation in galaxies, with the
corresponding behaviour in galaxy HODs (e.g., Brodwin et al.,
2013). This indicates that the processes that drive the pres-
ence of cold gas and ongoing star formation also drive SMBH
growth, as observed for field galaxies (see §4.2).

Recently, a number of studies have explored the AGN popu-
lation of protoclusters and comparable large-scale structures at
high redshift ($z > 2$). These works generally find substantial

<footnote>(of given $M_*$) that host radiatively inefficient AGN (LERGs) in fact remains roughly constant from $z \sim 0$ to $z \sim 1$, indicating that the same fuelling from hot halo gas operates in quiescent galaxies regardless of redshift. However, the space density of such quiescent galaxies drops rapidly over the same redshift interval, whereas star-forming galaxies become more prevalent and the fraction that host LERGs also increases, leading to the dominance of “star-forming LERGs” at high redshift.</footnote>
numbers of AGN in high-redshift systems, with evidence for
a (sometimes modest) enhancement in the AGN fraction com-
pared to similar field galaxies, with corresponding enhanced SF
(e.g., Lehmer et al., 2009, 2013; Tozzi et al., 2022; Monson
et al., 2023; Vito et al., 2024; Shah et al., 2024). This repre-
sents a reversal of the relationship between AGN and SF ac-
tivity for dense regions in the local Universe and continues a
trend previously found at lower redshifts (e.g., Martini et al.,
2013). An open question remains the extent to which the viri-
alisation of large-scale structures is important for suppressing
SMBH growth in the central regions, since high-redshift pro-
toclusters are typically less relaxed and gravitationally bound
compared to lower-redshift systems of similar total mass (e.g.,
Overzier, 2016). Nonetheless, the increasingly deep protoclus-
ter observations indicate that high-density structures are impor-
tant regions for SMBH growth in the early Universe.

4.4.2. *Spatial correlation measurements*

In addition to direct measurements of the incidence of AGN
in galaxy groups and clusters, there has been a great deal of
work using two-point correlation functions to measure the spa-
tial distribution of AGN and to infer their HODs; see Fig. 24 for
an illustration. These analyses can take the form of autocorrela-
tion of AGN samples (directly measuring their spatial correla-
tions; e.g., Myers et al. 2006; Eftekharzadeh et al. 2015; Com-
parat et al. 2023) or cross-correlation with galaxies, which has
the advantage of much larger statistical samples, but requires
an understanding of the clustering of the comparison galaxies
(e.g., Coil et al., 2009; Hickox et al., 2009).

The basic technique for measuring the halo mass from clus-
tering involves the computation of the galaxy or AGN *bias*; the
square of the bias is found by the relative amplitude of the cor-
relation function or power spectrum of galaxies, compared to
that for the underlying dark matter distribution. Using numer-
ical simulations of cluster formation and analytic theories for
halo formation via statistical collapse, formulae have been de-
veloped to convert a measurement of bias to a characteristic
*M<sub>halo</sub>* (e.g., Sheth et al., 2001; Tinker et al., 2010).

The simplest, but physically unrealistic, analysis assumes
that all objects in a clustering sample have the same M<sub>halo</sub>,
while others adopt a minimum M<sub>halo</sub> with the assumption that
the AGN occupy all haloes above this mass with a constant oc-
cupation fraction. More sophisticated analyses involve model-
ing the full HOD, which is a measure of the number of central
and satellite galaxies in the sample as a function of parent halo
mass (Berlind and Weinberg, 2002). In constraining the HOD,
the “two-halo” term (observed on scales larger than the size of
a single halo, or ≳ 1 comoving Mpc) sets the overall halo mass
scale, while the “one-halo” term (on scales ≲ 1 Mpc) deter-
mines the fraction of satellite galaxies. The full HOD is de-
scribed as the average number of central and satellite galaxies
as a function of halo mass.

The variety of ways to model the clustering of different halo
masses have led to challenges in comparing different measure-
ments of the bias or halo mass of AGN, so a measure of caution
is required in directly comparing results from different studies
(e.g., Aird and Coil, 2021). However, the *relative* clustering

and halo masses of different populations within a given analy-
sis technique tend to be robust.

Another related method of determining AGN halo masses
comes from weak lensing measurements. Lensing gives a mea-
surement of the integrated mass along the line of sight, and
so cross-correlation with AGN (or galaxy) sky positions, along
with knowledge of the source redshift distribution, can yield a
measurement of bias and halo mass that has largely indepen-
dent systematics compared to AGN and galaxy auto- or cross-
correlation functions (e.g., Bartelmann and Schneider, 2001;
Lewis and Challinor, 2006). This method has emerged in the
past decade due to the availability of high spatial resolution sur-
veys of galaxy shapes, for galaxy weak lensing, as well as high-
precision lensing maps of the CMB with ground-based CMB
experiments as well as the *Planck* satellite. Weak lensing using
galaxies and the CMB has been used for AGN samples identi-
fied in a range of wavebands including X-rays, optical, mid-IR,
and radio (e.g., Sherwin et al., 2012; Geach et al., 2013; DiPom-
peo et al., 2015; Comparat et al., 2023; Petter et al., 2024).

Two foundational results in AGN clustering set the stage for
progress in the past decade. First, optically-selected quasars
show weak, if any dependence in halo mass on luminosity and
redshift, being consistently found in halos of the small group
scale (M<sub>halo</sub> ~ 3 × 10<sup>12</sup> M<sub>☉</sub>; e.g., Croom et al. 2005; Myers
et al. 2007; Ross et al. 2009). Second, AGN selected in dif-
ferent wavebands demonstrate significantly different clustering
amplitudes, with increasing halo masses for AGN selected in
the mid-IR, optical and X-ray, and radio, respectively, with X-
ray and radio AGN found in halos of > 10<sup>13</sup> M<sub>☉</sub> (e.g., Hickox
et al., 2009). These results contributed to a picture in which
SMBHs grew, on average, in a radiatively efficient mode up un-
til halos reached the group scale, after which various processes
(e.g., merger activity, AGN feedback, emergence of a hot virial-
ized atmosphere) led to a decline in star formation and SMBH
growth (e.g., Hickox et al., 2009; Goulding et al., 2014).

In the past decade, a number of studies have computed clus-
tering of AGN selected in different wavebands and using a va-
riety of auto- and cross-correlation techniques as well as local
environmental density measures. These studies estimated both
the absolute halo masses of AGN, as well as bias relative to
“inactive” galaxies with similar properties such as stellar mass,
SFR, and redshift. On the whole, these studies indicate no obvi-
ous difference between AGN hosts and that of comparable “in-
active” galaxies, with results that are consistent across a wide
range of redshifts and AGN selection techniques (Mendez et al.,
2016; Yang et al., 2018a; Viitanen et al., 2019; Krishnan et al.,
2020). The typical halo mass of radio-selected AGN is con-
sistently higher than for radiatively-efficient AGN identified in
X-rays, optical, or mid-IR (e.g., Donoso et al., 2010; Devereux
et al., 2019; Petter et al., 2024); however, this in broad corre-
spondence with the halos of radio AGN host galaxies, which
are typically high-mass, passive systems (see §4.3) and is con-
sistent with a picture in which mechanical feedback from the
AGN emerges over cosmic time along with the passive galaxy
population. Robustly measuring the full AGN HOD remains
challenging due to degeneracy in HOD parameters and selec-
tion effects (e.g., Powell et al., 2024); various studies indicate a