Obscured Growth of Super Massive Black Holes at the Earliest Universe
Takuma Izumi, 1,2
1

National Astronomical Observatory of Japan, 2–21–1 Osawa, Mitaka, Tokyo 181–8588, Japan2
Department of Astronomical Science, The Graduate University for Advanced Studies, SOKENDAI, 2–21–1 Osawa, Mitaka, Tokyo
takuma.izumi@nao.ac.jp

Abstract
I discuss the importance of high resolution and high sensitivity observations of cold molecular gas at the central∼ 100 pc scale region (circumnuclear disk = CND) of a precursor of z > 6 − 7 quasars. The usual suspect is a
gas-rich starburst galaxy that hosts an obscured active galactic nucleus/AGN (such as an ultra-luminous infrared
galaxy = ULIRG). Therefore here I explore a feasibility of 100 pc resolution observations of low-J CO and [C I]
emission lines toward a z = 10 ULIRG. The planned observations will tell us the CND-scale molecular gas mass,
the nature of the obscured heating source (AGN vs starburst), gravitational stability of the CND (potential triggerof mass accretion), and even the black hole mass itself. A synergetic works with future X-ray satellites will also
reveal its Eddington ratio. Thanks to the superb capability of ngVLA, we can conduct this ambitious program witha modest investment of the telescope time.
Key words: galaxies: active — galaxies: evolution — galaxies: ISM — galaxies: nuclei

1. Introduction
The discovery of a tight correlation between the masses ofcentral supermassive black holes (MBH ) and those of galactic bulges (Mbulge ) or the stellar velocity dispersions in thelocal universe (e.g., Magorrian et al. 1998; Marconi & Hunt
2003; Kormendy & Ho 2013) strongly suggests that the formation and growth of supermassive black holes (SMBHs) andtheir host galaxies are intimately linked, and the two undergo a
co-evolution. Although the detailed mechanism by which thecorrelation arises is unclear, theoretical models suggest that radiative and kinetic feedback of active galactic nuclei (AGNs)
connected to the merger histories of galaxies play a pivotal role(e.g., Di Matteo et al. 2005; Hopkins et al. 2008). Detectionsof galaxy-scale massive AGN-driven outflows (e.g., Nesvadba
et al. 2008; Maiolino et al. 2012; Cicone et al. 2014), likely
elevated AGN fraction in interacting/merging galaxies (e.g.,
Silverman et al. 2011; Goulding et al. 2018), as well as theremarkable similarity of global star formation and SMBH accretion histories (Madau & Dickinson 2014 for a review) would
support this evolutionary scheme.
It is intriguing that recent high-resolution simulation works
based on the above scheme suggest that even a quasar at z =7 would show a consistent MBH –Mbulge relation to the local
one, once we properly assess the mass of the host galaxy (Lupiet al. 2019; Marshall et al. 2020). From the observational side,z > 6 quasar host galaxies have been extensively studied atlong-wavelength (e.g., submm) as these galaxies are hard to detect at rest-frame UV to optical due to the outshining brightnessof the nuclei (Mechtley et al 2012). Previous high resolution [CII] emission line observations, mainly performed by using the
Atacama Large Millimeter/submillimeter Array (ALMA), provided basic star-forming nature and dynamical mass (Mdyn )of those host galaxies (e.g., Wang et al. 2013; Venemans etal. 2016). These works revealed that z > 6 quasars are typically hosted by vigorous starburst galaxies (star-formation rate

−1
SFR >
∼ 100 − 1000 M⊙ yr ), and have over-massive supermassive black holes (SMBHs) relative to the local co-evolution
relation (i.e., their MBH is ∼ 10× more massive than what is
expected from the local relation)1 . However, previous observations would be highly biased toward the most luminous quasars(1450Å magnitude M1450 < −26), which correspond to themost massive SMBH population that accordingly bias the observed shape of the z > 6 co-evolution relation (Lauer et al.
2007).
Recent ALMA observations toward z ∼ 6 − 7 optically lowluminosity quasars (M1450 > −25), that typically have lower
mass MBH than the luminous ones, however, have started toshow that these low-luminosity (= less-biased) objects tend
to follow the local co-evolution relation (Izumi et al. 2018,2019): this is consistent with the hydrodynamic model predictions indeed (Marshall et al. 2020). It is noteworthy that such
low-luminosity quasars are nowadays, and will be, routinely
discovered from wide area and deep optical imaging surveys
by using, e.g., Subaru Hyper Suprime-Cam (HSC) and Vera
C. Rubin Observatory (VRO). With this momentum, we will
sooner or later be able to obtain a census of star-forming anddynamical nature of quasars even at z ∼ 6 − 7.

2. Hunt for a dusty precursor
The emergence of the quasars, as well as the co-evolution relation already at z ∼ 6 − 7, require a quite rapid process for the
early galaxy/SMBH formation and evolution. The usual suspect is mergers of gas-rich galaxies, which can drive a growthof massive galaxies and SMBHs with a reasonably short timescale of <∼ 1 Gyr (e.g., Hopkins et al. 2008). Such a mergerinduced active galaxy appears as a dusty starburst known as
an ultra-luminous infrared galaxy (ULIRG) in the local uni1

We need to equate Mdyn to Mbulge as we do not have any information on
their host galaxy stellar light until the operation of the James Webb Space
Telescope.

2[Vol. ,

Fig. 1. An expected evolutionary sequence from a gas-rich merger toan optical quasar (figure modified from Hopkins et al. 2008). Given
the already known existence of quasars at z ∼ 7.5, we will try to characterize their precursors at further higher redshifts such as z ∼ 10.
Table 1. Target Emission Lines

Transition
CO(1–0)CO(2–1)CO(3–2)[C I](1–0)[C I](2–1)

νrest
(GHz)
115.271230.538345.796492.161809.342Eu /kB
(K)
5.5
16.6
33.2
23.6
62.5

ncr
(cm−3 )
2.1 × 103
1.1 × 104
3.6 × 104
1.2 × 103
3.1 × 103verse and/or a submillimeter galaxy (SMG) at higher redshifts.
Therefore, it is intriguing to uncover such ULIRG/SMG-classgalaxies at a significantly earlier universe than z ∼ 7.5 (current redshift-record of quasars, Wang et al. 2021), e.g., z ∼ 10
(Figure 1), and characterize their properties.
However, such starburst galaxies at z > 7 must be quite rare:a recent observational estimate on IR luminosity function at
4.5 < z < 6 shows a number density of ULIRG-class objects−4−3−1(1012 < LIR /L⊙ < 1013 ) is as low as <∼ 10 Mpc dex(Gruppioni et al. 2020). We thus need to perform an ultra-widearea and deep survey at first to identify such objects. Next
generation 50 m-class single dish telescopes that have ∼ deg2
field-of-view (e.g., Large Submillimeter Telescope = LST) will
be able to conduct deep spectroscopic surveys over a wide areaon the sky, with the aid of ultra-wide bandwidth superconducting spectrometers (Endo et al. 2019).
3. Extensive characterization with ngVLA
I hereafter discuss possible follow-up observations for those
newly detected starburst systems at z > 7.5, with mainly focusing on a z = 10 object (1′′ = 4.2 kpc) as an educational example(Figure 1). Suppose firstly that there is a hidden AGN at its nucleus behind the curtain of dust, which would be experiencing
a rapid mass accretion given the blessed availability of gas. As
explained later, we will attempt to measure a cold molecular
gas mass (Mmol ) of the system. Hence our target emission lines(Table 1) may include the low-J 12 CO lines (classical proxies
of Mmol ; Bolatto et al. 2013), and atomic carbon fine structurelines ([C I]) that are nowadays considered to be another goodproxies of Mmol in various environments, particularly in highredshift galaxies (e.g., Papadopoulos & Greve 2004; Israel etal. 2015; Jiao et al. 2017; Valentino et al. 2018): these emission lines of a z >∼ 8 galaxy fall well into the frequency coverageof the next generation Very Large Array (ngVLA, Figure 2).
Thanks to the extremely high angular resolution and sensi-

Fig. 2. Observed frequencies of our target emission lines (12 CO and[C I ]) as a function of redshift. Shaded regions indicate the frequency
coverage of ngVLA. As shown here, we can observe all these lines
from galaxies at z ∼ 10, for example.

tivity provided by ngVLA, we can investigate the following key
topics regarding the obscured AGN.
(1) How massive is a circumnuclear gas disk? A number of theoretical works and local observations claim an importance of sub-kpc-scale circumnuclear gas disk (CND) formed
around an AGN as a direct reservoir of fuel onto the SMBH
(e.g., Thompson et al. 2005; Kawakatu & Wada 2008; Izumi
et al. 2016). A key physical process here to drive gas inflows
would be a gravitational instability of the disk (Toomre 1964).
We will assess this by measuring the gas mass and dynamical
properties of the CND. The low-J CO lines and the [C I] lines
can be used to measure Mmol , if we have proper conversion
factors from the line luminosity to Mmol for AGN environments. We should really care about this point as AGN can alter
the chemical composition of the surrounding gas (see the item(2) in the following), which would make the conversion factors
significantly different from what measured for systems without
AGN (e.g., Bolatto et al. 2013; Jiao et al. 2017).
Such proper conversion factors can be estimated from extensive analyses of high resolution line emission data and stellar
mass distribution data of nearby active galaxies. For example,
Izumi et al. (2020) modeled the CO(2–1) and the [C I](1–0) velocity fields of the type-1 Seyfert galaxy NGC 7469 to obtainradial profiles of dynamical mass (Mdyn). They subtracted astellar mass distribution measured by using the Hubble Space
Telescope from this Mdyn to directly obtain Mmol radial distribution as a residual. This procedure directly provides theconversion factors after dividing the Mmol by L′CO or L′[CI] .
Systematic applications of this method to nearby AGNs by using ALMA, for example, will deliver the conversion factors ina statistical manner: we can therefore apply them to high redshift objects in the ngVLA era.
(2) How to show the existence of an obscured AGN?
Almost by definition, it is virtually impossible in an ULIRGclass object to detect the light directly emerging from the AGN
at rest-frame UV-to-optical wavelengths due to severe dustextinction (i.e., difficult to reveal the existence of an obscured

3(TB unit)

No. ]

AGN

Starburst(TB unit)

Fig. 3. [C I ](1–0)/12 CO(2–1) vs [C I ](1–0)/13 CO(2–1) line flux ratios(TB unit) of various nearby galaxies (figure modified from Izumi etal. 2020). High resolution (∼ 130 pc) ALMA observations toward the
CND and the surrounding starburst region of NGC 7469 (nearby type-1
AGN) are shown by the filled symbols. The open symbols indicate theliterature data taken by single dish (SD) telescopes, color-coded as afunction of their nuclear types (AGN, starburst, quiescent). One cansee that the line ratios are elevated in AGN systems, particularly whenobserved at a high-resolution that selectively probes the CND (XDR).

AGN). However, we will overcome this issue with the combination of the target emission lines. We emphasize that one
very unique point of AGNs, as compared to star-formation, isthat AGNs are a powerful source of hard X-ray radiation in theuniverse. Hence X-ray dominated regions (XDRs, Maloney
et al. 1996; Meijerink and Spaans 2005), where gas physical and chemical properties are governed by the X-ray radiation, should form around them. It is striking in this context that Izumi et al. (2020) found a markedly elevated [CI](1–0)/CO(2–1) and/or [C I](1–0)/13CO(2–1) line ratios at the∼ 100 pc-scale CND of NGC 7469 as compared to starburst
regions of the same galaxy, as well as to local starburst galaxies (Figure 2). According to non-LTE calculation, they concluded that this enhancement is due to an elevated C abundance relative to CO, which is a natural consequence in an
XDR as CO is efficiently dissociated there. With the superb
angular resolution of ngVLA, we can properly map the central∼ 100 pc regions even in z ∼ 10 galaxies at which we expect
to see XDR signatures (Schleicher et al. 2010), and capture
this chemically-driven line enhancement as an extinction-freeindicator of AGN.
(3) What is the characteristic feature of that AGN?
Synergies with future projects. If an obscured AGN indeed corresponds to an early evolutionary phase of cosmic
SMBH growth as expected in the merger-induced evolutionary models (e.g., Hopkins et al. 2008), that AGN would
have a smaller MBH and a higher Eddington ratio (probably
a Super-Eddington ratio) than previously-identified normal optical quasars at z ∼ 6 − 7 (MBH ∼ 109 M⊙ , Eddington ratio∼ 0.1−1, e.g., Onoue et al. 2019). We will try to measure thesebasic properties of obscured AGNs at z ∼ 10 with ngVLA.

Recent high resolution ALMA observations of CO lines in
the local universe have demonstrated the great power of coldgas dynamics measurement to constrain MBH by capturing
the Keplerian steep rise in a rotation curve (e.g., Davis 2014;
Onishi et al. 2015; Nguyen et al. 2020). We may naturally
apply this methodology to higher redshift objects with ngVLA.
However, as most of the previous cold gas works on MBH measurement were conducted toward relatively quiescent galaxies,
one caveat would be again the XDR effect on the gas composition (CO dissociation). We can overcome this issue by focusing on the [C I] lines instead, as shown in Figure 4, which isthe case of NGC 7469, a luminous AGN in the nearby universe. One can clearly see that the [C I](1–0) line is more
centrally concentrated than CO(1–0), which is a natural consequence of the XDR effect. Therefore, even if the XDR effectmatters in obscured AGNs at z ∼ 10, we will still be able toconstrain their MBH by observing [C I] lines. Note that we doneed stellar mass distribution data to better model the gravitational potential of the galaxy. This will become feasible once
next generation large optical/NIR telescopes (e.g., European
Extremely Large Telescope = E-ELT, Thirty-Meter Telescope= TMT) start to operate.
Synergies with future X-ray satellites will be of our interest
as well. The ESA’s Athena satellite and the NASA’s Lynx satellite planned in 2030s will efficiently sample the rest-frame > 10
keV photons with extremely high sensitivities that will reachz ∼ 10 universe. This high-energy range does not significantly
suffer from the Compton thickness effect, hence probes the intrinsic AGN luminosity in a least biased manner. Combinedwith MBH estimated by ngVLA, we will be able to assess the
Eddington ratio even for an obscured object at that high redshift.
4. Feasibility?
In order to dynamically measure the MBH , as well as to selectively probe the XDR-like spatial scales, we here suppose toperform a 100 pc (0′′ .024) resolution CO and [C I] emission
line observations toward a z = 10 galaxy (with an obscuredAGN). Note that this resolution is still worse than the typicalsphere of influence (SOI) size of a 108 M⊙ SMBH (several
×10 pc), but the SOI criterion does not necessarily apply forcold gas dynamics-based MBH measurements as shown in previous works (Davis 2014; Nguyen et al. 2020).
Suppose that there is 4 × 109 M⊙ molecular gas within
this 100 pc aperture, which is roughly the case of the nearby
ULIRG Arp 220 (Scoville et al. 2015). If we apply the CO(1–0)-to- and [C I](1–0)-to-molecular mass conversion factors derived at the XDR scale of NGC 7469 (Izumi et al. 2020),
namely αCO = 4.1 and αCI = 4.4 M⊙ (K km s−1 pc2 )−1 ,we obtain corresponding velocity-integrated line fluxes of
S∆VCO10 = 4.1 × 10−4 (at νobs = 10.5 GHz) and S∆V[CI]10 =6.9 × 10−3 Jy km s−1 (at νobs = 44.7 GHz), respectively(z = 10). By assuming a CO(2–1)/CO(1–0) ratio of ∼ 7 measured at the central ∼ 100 pc of NGC 7469 (Izumi et al. 2020),
as well as a [C I](2–1)/[C I](1–0) of ∼ 2 found in the galaxywide scale of ULIRG-class objects (Jiao et al. 2017) 2 , we2Currently we do not have a good measurement of this ratio at the central

4[Vol. ,NGC 7469

CO(1-0)[CI](1-0)

Fig. 4. Position-velocity diagrams of (a) CO(1–0) and (b) [C I ](1–0) cut along the kinematic major axis of NGC 7469, a luminous type-1 AGN (figure
adopted from Nguyen et al. 2021). One can clearly see that [C I ](1–0) is more concentrated toward the center (AGN) than CO(1–0), which is a clear
consequence of the XDR effect (CO dissociation).

then obtain S∆VCO21 = 2.8 × 10−3 (at νobs = 21.0 GHz) andS∆V[CI]21 = 1.4 × 10−2 Jy km s−1 (at νobs = 73.6 GHz).
Although it is highly dependent on the adopted conversion factors, the potentially brighter line flux of [C I](2–1) suggests that
it would be more prioritized than CO(2–1) to probe the gas dynamics. However, CO(2–1) can still be used to take a line ratio
to diagnose the hidden energy source of the system (Figure 3).
If we further assume a Gaussian line profile (FWHM = 300
km s−1 ) for both CO(2–1) and [C I](2–1), we will obtain• CO(2–1): S/N ∼ 5 at the Gaussian profile peak, withan on-source time of 25 hr and a velocity resolution of
dV = 100 km s−1 ,• [C I](2–1): S/N ∼ 15 at the Gaussian profile peak, withan on-source time of 30 hr and dV = 50 km s−1 ,according to the ngVLA key performance matrix. The S/N ofthe [C I](2–1) observation will be fine to perform dynamical
modelings of the observed velocity field. Although this is still asignificant investment of the telescope time for a single object,
I would claim it is worth conducting given the obvious extreme
rarity of z = 10 ULIRG-like galaxies.
One would care about the additional heating effect and thebrightness contrast effect due to the high temperature (30 K at
z = 10) cosmic microwave background (CMB) radiation (daCunha et al. 2013). This is indeed a nightmare for normal starforming and/or quiescent galaxy observations. However, nowthat we focus on a very active galaxy, we would naively expect
both the gas density and the kinetic temperature are high, i.e.,
gas excitation is reasonably high. Therefore, if the line excitation temperature of these lines exceeds 100 K, for example, we>
can recover >∼ 75% and ∼ 80% of the intrinsic line fluxes ofCO(2–1) and [C I](2–1) against the CMB, respectively, which
do not significantly affect the detectability of these lines.
100 pc-scale of external galaxies. Near-future ALMA observations will
constrain this. We thus keep using the galaxy-scale (>kpc-scale) measurement in this document.

5. Summary
Given the existence of quasars at z > 6 − 7, as well as
the rapid emergence of the co-evolution relation already atz ∼ 6 − 7 (e.g., Izumi et al. 2018, 2019), it is urgent to identify and characterize their precursors at further higher redshifts.
Possible candidates include a dusty ULIRG-like galaxy formed
by a major merger of gas-rich galaxies, which will be uncovered by future wide-area single dish spectroscopic/imaging surveys. In this document, I briefly discuss high resolution (100pc) CO and [C I] observations toward such a dusty galaxy at
z = 10 that harbors an obscured AGN. Thanks to the superb
capability of ngVLA, we will be able to diagnose the obscured
heating source (AGN vs starburst), to measure the CND-scale
molecular gas mass, to assess the gravitational stability of thatdisk (a likely key physical process to trigger inflows), andto estimate the SMBH mass. We can further constrain the
Eddington ratio of that SMBH as a synergy with future X-ray
satellites. Therefore, ngVLA has a great capability to explore
the obscured phase of SMBH growth even at that earliest universe.
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