Resolving the Parsec-scale Feeding and Feedback Flows of Ionized Gas
around Active Galactic Nuclei
Takuma Izumi, 1,2
12

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

Abstract
Recent high resolution cold molecular/atomic gas observations have started to reveal detailed spatial structuresand dynamics of ∼ 100 pc-scale circumnuclear disks (CNDs) around active galactic nuclei (AGNs). In order toextend such efforts to further inner regions around supermassive black holes, where gas is primarily ionized dueto the harsh AGN radiation, we here investigate the detectability of mm-HI recombination lines (RLs) with the
New Generation Very Large Array (ngVLA). We found that subparsec-scale dense inflows (density ne ∼ 106 cm−3 ,temperature Te ∼ 2 × 104 K) are readily detectable with a reasonable on-source observing time (<∼ 10 hr), allowing
us to study detailed spatial distributions, dynamics, and potentially inflow rates, of ionized gas. Rather diffuser andhotter subparsec-scale outflows (ne ∼ 102 cm−3 , Te ∼ 106 K) are impossible to detect even with ngVLA, but there
is still a chance to detect a dense one (on-source time ∼ 15 hr), which will tell us spatial/dynamical connectionsof nuclear ionized winds and surrounding cold material. The proposed experiment here will play a vital role tocomprehensively understand the multi-phase nature of AGN feeding and feedback.
Key words: galaxies: active — galaxies: evolution — galaxies: ISM — galaxies: nuclei
1. Introduction
The mass accretion onto a supermassive black hole (SMBH;
6
with a mass of MBH >∼ 10 M⊙ ) is commonly ascribed to generate the enormous amount of energy that is observed as anactive galactic nucleus (AGN) or a quasar (e.g., Alexander &Hickox 2012). AGN can launch fast nuclear outflows that propagate to the host galaxy-scale, clear out the cold gas, and would
eventually quench star-formation (e.g., King & Pounds 2015;
Veilleux et al. 2020). This negative feedback is incorporated
to virtually all cosmological galaxy evolution models to reproduce the tight correlation between MBH and bulge scale properties of galaxies observed at the local universe (Kormendy &ho 2013 for a review). Hence investigating detailed feeding
and feedback mechanisms of AGNs is vital to understand thecosmological evolution of SMBHs and galaxies.
Past observations over all electromagnetic wavelength revealed a multi-phase (i.e., ion, atom, and molecule) and complex nature of circumnuclear gas structures at the central <∼ 100
pc scales of AGN-host galaxies (Figure 1). At the immediate
vicinity of an AGN, a highly ionized region of fast moving
gas is formed (broad line region = BLR), which is further surrounded by a dusty/gaseous obscuring structure (torus) that determines the optical spectroscopic appearance of the AGN (unification model; Antonucci 1993). A more extended (over tensof parsecs to a few kiloparsecs) narrow line region (= NLR)
is illuminated by the AGN, which therefore has a characteristic spectral features (e.g., line ratios) as compared to normallystar-forming galaxies (e.g., Baldwin et al. 1981).
Recent high resolution cold molecular gas observations
by using the Atacama Large Millimeter/submillimeter Array(ALMA) started to reveal that AGNs are usually embeddedin ∼ 10 − 100 parsec (pc) scale dense molecular gas disks(circumnuclear disk = CND; e.g., Izumi et al. 2016a, 2018;
Imanishi et al. 2018; Combes et al. 2019). This CND canwork as a direct reservoir of fuel for the central SMBH. Hencea dense gas flow (= inflow), probably through the mid-plane of
the CND, would have triggered the AGN. At the NLR-scale,infrared integral-field spectroscopy detected extended outflows
traced by some recombination lines and high-ionization lines
including [SiVI], [CaVIII] (e.g., Müller-Sánchez et al. 2011).
These observational facts are well consistent with recent hydrodynamical simulations (e.g., Wada et al. 2016, 2018), in
which multi-phase gas flows are responsible for the complex
circumnuclear structures.
2. Limitations of the Past Observations
Given the growing success of ALMA, it should be surely
feasible to trace molecular gas flows down to ∼ 1 pc-scales ofnearby AGNs (see early examples in, e.g., Impellizzeri et al.
2019; Imanishi et al. 2020). However, probing further inward
regions of AGNs such as 0.1 pc-scale has a fundamental difficulty. That is, AGNs are in principle powerful sources of X-ray
radiation, which easily dissociates and ionizes the surrounding
dense cold medium to hot plasma (e.g., Maloney et al. 1996;
Namekata et al. 2016). Thus we need to perform ionized gas
observations to ultimately probe gas inflows to the SMBHs. In
the case of our Galactic Center, Tsuboi et al. (2017) indeed detected pc-scale ionized gas flows toward the Sgr A* traced by
H42α recombination line and 100 GHz continuum emission.
Regarding the outflows, there is ample evidence of fast
molecular ones in AGN-host galaxies, most of which are considered to be AGN-driven (e.g., Cicone et al. 2014; Lutz
et al. 2020). The molecular phase carry the bulk outflowing mass among the other gas phases (Fluetsch et al. 2019),

2[Vol. ,
Outﬂow

Log (z/pc)

Narrow Line Region

Color: 350GHz continuum+2

Contour: Hα

+1
0
-1

CO(3-2)-2

Broad Line Region
Torus/Circumnuclear Disk

-3

Inﬂow

accretion disk

-2

SMBH

-1

0

+1

Log (r/pc)+2-3

Fig. 1. A schematic view of the circumnuclear gas structures around an
AGN (actively mass accreting SMBH). Recent high resolution observations already resolved the structures at >∼ 10 pc scales: examples of theCircinus galaxy are shown on the right panels (Figures adopted from
Izumi et al. 2018). However, the detailed/real structures at < 0.1 − 1
pc scales remain unclear: we will tackle this small scale with ngVLA.

which has therefore a crucial impact on galaxy evolution. One
possible physical origin of molecular outflows is that the hot,high velocity wind launched from regions close to the AGN
hits/accelerates the surrounding molecular clouds of the hostgalaxy (e.g., Gaspari & Sadowski 2017). However, if the dense
molecular gas is settled in a CND and the hot wind is launched
to a perpendicular direction relative to this disk (Figure 2),
there would be little chance for the hot wind to interact withthe surrounding dense gas. In addition, numerical simulations
showed that such a hot wind-to-dense gas interaction actuallydestroys the molecular gas before it is accelerated to the observed outflow velocities of several 100 to 1000 km s−1 (e.g.,
Scannapieco & Brügen 2015; Schneider & Robertson 2017).
An alternative scenario was proposed by Richings & FaucherGiguère (2018), in which molecules form in situ within AGNdriven winds due to efficient radiative cooling. In this casethere is probably no need to align high velocity winds anddense gas. To better understand the formation of molecular outflows, observations of the distributions and dynamics of multiphase winds are therefore essential.
3. High Resolution Mappings of mm-HI RecombinationLines with ngVLA
3.1. Expected Line Flux for Inflows
Considering the background in § 2, we claim that it is imperative to investigate spatial distributions and dynamics of hot
ionized gas flows at the close vicinity (r <∼ 1 pc) of AGNs.
To this end, mm/submm HI recombination lines (RLs) works
powerfully because:• These lines directly trace ionized gas.
• We can achieve an extraordinary high resolution (<∼ 5−
10 mas scale) and sensitivity by using ngVLA, which arenecessary to probe the very central regions of AGNs.
• Shorter wavelength (e.g., MIR) RLs will suffer a certain
amount of dust extinction. Milli-arcsec resolution spectroscopy is impossible at these wavelengths as well.

Fig. 2. Simulated distributions of multi-phase gas flows around an
AGN (MBH = 2 × 106 M⊙ , Eddington ratio = 0.2) adopted from
Wada et al. (2016). Colors indicate the gas volume density and the arrows represent velocity vectors of the flows. Each phase gas occupies
a different volume. In this simulation, high velocity winds are only
visible in the ionized phase, which travel almost perpendicular to themolecular disk.

No. ]

3
Table 1. Assumed Conditions for Inflows

Target Linene = np (cm−3 )Te (K)
l.o.s. Pass Length (pc)Line width (FWHM, km s−1 )He+ /H+ abundance ratio

H42α (at 85.69 GHz)
106
2 × 104
0.05
100
0.08

• Longer wavelength (cm) RLs will easily become optically thick and sometimes suffer maser amplification.
These will simply complicate the situation.
We first investigate a detectability of mm RLs in (sub)pcscale inflows by focusing on H42α at νrest = 85.69 GHz as arepresentative example. Results can be applied to other RLs
with comparable principal quantum numbers.
We now consider physical conditions summarized in Table
1. The electron density (which is identical to the proton density under the fully ionized condition) is set to ne = 106 cm−3 ,and the electron temperature is set to Te = 2 × 104 K. These
are characteristic to a relatively high density (mid-plane) region of a CND at r <∼ 1 pc as found in very high resolutionhydrodynamic simulations of Namekata & Umemura (2016).
The line-of-sight (l.o.s.) length of this dense part measured
from the face-on angle is ∼ 0.05 pc. Although Namekata &Umemura (2016) revealed a geometrically much thicker volume of ionized gas (height ∼ 1 pc) with a considerably higher
Te of ∼ 106−7 K, we neglect this volume as a simulated gas
density there is much smaller (< 100 cm−3 ), which therefore
only has a minor contribution to a line intensity. For the caseof (ne , Te ) = (106 cm−3 , 2 × 104 K), a departure from thelocal thermodynamic equilibrium (LTE) should be negligible
for a RL with a principal quantum number of >∼ 40 (Walmsley
1990). Hence we can easily compute the line intensity (brightness temperature unit) by using the l.o.s. emission measure(EM) and a line opacity, which are expressed as: Z ne   np  dl
EM ≡
(1)
cm−3
cm−3
pc

−1
 −5/2 
EM
∆ν
3 Te
(2)
τRL = 1.92 × 10
K
cm−6 pc
kHz
where ∆ν accounts for the line width (100 km s−1 = 28583
kHz). With the numbers in Table 1, we obtain τRL = 0.059.
However, we should also consider the underlying free-freecontinuum emission as this is also emitted from the same gas
volume where the line emission originates. By using an appropriate Gaunt factor for free-free transitions, a continuum opacity can be expressed as −1.35 


Te
EM
ν −2.1
τC = 8.235×10−2
(3)
K
GHz
cm−6 pc
Note that this continuum EM includes a contribution from He+
ion, which is not the case for HI RLs. This time we obtain amoderate opacity of τC = 0.606.
With the above quantities, we can compute the RL intensity
and the continuum intensity as

Table 2. Assumed Conditions for Outflows

Target Linene = np (cm−3 )Te (K)
l.o.s. Pass Length (pc)Line width (FWHM, km s−1 )He+ /H+ abundance ratio

H42α (at 85.69 GHz)
102
106
1.0
300
0.08

Table 3. Assumed Conditions for Very Dense Outflows

Target Linene = np (cm−3 )Te (K)
l.o.s. Pass Length (pc)Line width (FWHM, km s−1 )He+ /H+ abundance ratio

H42α (at 85.69 GHz)
104
2 × 104
1.0
300
0.08

TRL = Te (1 − e−(τRL+τC ) )TC = Te (1 − e

−τC

),

(4)
(5)

whereas a continuum-subtracted line intensity as∆TRL = TRL − TC

(6)

For the case of H42α we now obtain ∆TRL ≃ 630 K. This isfairly a high value thanks to the high Te and modest opacities
explored here. Note that Eq.(6) indicates that no RL can beobserved if the continuum emission is heavily optically thick.
This is particularly the case for BLR clouds, in which we ex10
−3pect a tremendously high gas density of ne >∼ 10 cm (seealso Izumi et al. 2016b).
3.2. Expected Line Flux for Outflows
We next consider the (sub)pc-scale outflows by assuming thephysical condition in Table 2. The electron density and temperature are the characteristic ones found at several pc-scale
extended windy regions in hydrodynamic simulations of Wadaet al. (2016). With these numbers, however, we found thatboth the RL and the underlying free-free continuum are extremely optically thin (τ ≪ 10−9 ), which results in an emergent line flux of almost zero. This is therefore not detectable
at all even with ngVLA. Possible exception is very dense outflows (e.g., Kawaguchi et al. 2018) such as (ne , Te ) = (104cm−3 , 2 × 104 K; Table 3), although simulations usually do
not find such dense ionized outflows at the central pc-scales ofgalaxies. Nevertheless, in this case we expect a line intensityof ∆TRL ∼ 1 K.
3.3. High-resolution Observations with ngVLA
Given the brightness, we first consider requirements (resolution, sensitivity, etc.) for inflow observations. Tsuboi et al.
(2017) mapped H42α distribution around Sgr A* (Figure 3),
which revealed spiral flows with a length of ∼ 1.5 pc and awidth of ∼ 0.2 − 0.3 pc. Therefore, we request spatial resolutions better than 0.3 pc for our study so that we can spatiallyresolve the ionized inflows in nearby AGNs. Note that it is4[Vol. ,
~1.5 pc

Physical Resolution
Angular Resolution
Velocity Resolution
Targeted 1σ
On-source time

Sgr A*

Fig. 3. Velocity channel map (80 km s−1 ) of the Galactic Center region
traced by the H42α recombination line (color), overlaid with the 100
GHz continuum emission contours (adopted from Tsuboi et al. 2017).
One can identify spiral flows of ionized gas with a spatial extent of∼ 1.5 pc, traveling down to the Sgr A*.

NGC 1433
NGC 1566

0.2 pc
5.8 mas (D = 7 Mpc)
10 km s−1
60 K
∼ 7.5 hrdistributions in detail, as well as to assess ionized gas dynamics there with dynamical modelings that isolate non-circularmotions from rotation + random dispersion motions. We cantherefore answer some key questions including:Color: H42αContour: 100 GHz continuum

M51

Table 4. Fiducial Feasibility Estimation (M51; inflow)

NGC 1068
NGC 1097

NGC 4826

Fig. 4. Expected physical resolutions for three example ngVLA angular resolutions (3, 5, and 10 mas) as a function of the distance to the target galaxy. The shade region indicates our acceptable resolution range(i.e., <∼ 0.3 pc) to properly map ionized flows (see also Figure 3). Somefamous AGNs observable from ngVLA are marked.

also plausible that AGNs have more extended ionized flows(≫ 1 pc) given the fact that Sgr A* is a much less active system than typical Seyfert galaxies by orders of magnitude: inthis case we can readily resolve the flows by the 0.3 pc resolution. We show in Figure 4 this spatial resolution limit as
a function of the distance (D) to a target galaxy. Thanks tothe extremely high angular resolutions (< 10 mas) provided by
ngVLA, we can surely fulfill this resolution requirement up toD ∼ 15 Mpc. Note that this cosmic volume includes some representative AGNs in the nearby universe such as NGC 1068,
NGC 1097, and M51.
For example, suppose M51 as our fiducial target AGN (Table4), which is located at D ∼ 7 Mpc. Due to this proximity,
we can obtain, e.g., 0.2 pc resolution with a 5.8 mas beam.
According to the ngVLA Key Performance Metrics, we willobtain 1σ sensitivity of 60 K with only ∼ 7.5 hr on-source integration. This allows a ∼ 10σ detection of the ∆TRL of H42α(line center). This high S/N should be sufficient to map the gas• Can we see spiral flows like those in Sgr A*?
• Can we see non-axisymmetric gas distribution at the final
pc-scale region, which is essential to cause gravitational
instability?
• What is the spatial connection of these ionized flows to
the larger scale cold gas distributions probed by, e.g.,
ALMA?
• What is the 3D geometry of the flows? We can constrain
this (scale height/length ratio = aspect ratio) from the decomposed dispersion-to-rotation velocity ratio.
• How fast are the inflows?
• What is the inflow rate (≡ Ṁ )?
Note that the flux of RLs depends on the line emissivity and
EM as long as the emission is (at least moderately) opticallythin. Here the line emissivity is almost independent of ne for
RLs considered here (Scoville & Murchikova 2013). That is,once we constrain the 3D geometry, we can roughly computene from EM, which would deliver the 1st-order estimation ofṀ when combined with information of the spatial structure
and the inflow velocity. The proposed observation here will
finally bridge the innermost part of a molecular CND and a
BLR/accretion disk scale, enabling a thorough discussion on
dynamical gas flows at the central < 10 pc scales of AGNs.
For the case of vey dense outflows (Table 3), it is still notpractical to request mas-scale resolutions, which will inflate
the observational time. Even for ngVLA, this experiment is′′only feasible for rather modest angular resolutions of >∼ 0 .1,
which corresponds to >∼ 3.4 pc at the distance of M51. With
this reduced resolution, we may obtain ∼ 10σ detection of theoutflowing H42α with a ∼ 15 hr on-source time (dV = 30 kms−1 ). This is still worth doing to reveal the feedback properties
at the central < 10 pc regions of AGNs. By assessing the dynamical properties and mapping the intensity distribution, we
would like to answer these questions:• What is the spatial connection of the outflowing ionizedgas and the surrounding cold gas?
• In particular, can we see the site at which ionized outflows indeed contact cold molecular/atomic gas?
• Is the outflow velocity sufficiently high to escape fromthe gravitational potential of the SMBH?
4. Summary
In this article we investigated the detectability of mm HI recombination lines (example case = H42α) at the close vicinity

No. ](< 1 pc) of AGNs. This is essential to comprehensively understand the feeding and feedback of AGNs, primarily because
cold gas (molecule + atom) must be heated and ionized by thecentral AGN at that innermost part, i.e., cold gas observations
cannot probe such core regions in principle. This experimentrequires quite high angular resolutions (mas-scale) and sensitivity, which are only achievable by ngVLA.
Regarding the subpc-scale ionized inflows, we found thatthey are readily detectable in nearby AGNs (D <∼ 15 Mpc).
We can therefore investigate detailed ionized gas distributions,dynamics including inflow velocities, inflow rates, as well as
physical connections to larger-scale cold gas disks probed by,e.g., ALMA (and ngVLA itself). On the other hand, due tothe likely less-dense and hotter nature of the gas therein, it isquite difficult to detect (relatively diffuse) ionized outflows at
the central pc-scale region. Ionized outflows would only be detectable if the gas density is sufficiently high and the temperature is modest. Even so, we may be able to investigate a slightly
larger scale (< 10 pc) ionized outflow properties, which still
has a substantial impact on feedback studies.
References
Alexander, D. M. & Hickox, R. C., 2012, New Astronomy Reviews,
56, 4Antonucci, R., 1993, ARA&A, 31, 473
Baldwin, J. A., Phillips, M. M., & Terlevich, R., 1981, PASP, 93, 5Cicone, C., et al., 2014, A&A, 562, A21
Combes, F., et al., 2019, A&A, 623, A79
Fluetsch, A., et al. 2019, MNRAS, 483, 4586
Gaspari, M., & Sadowski, A., 2017, ApJ, 837, 149
Imanishi, M., et al., 2018, ApJL, 853, L25
Imanishi, M., et al., 2020, ApJ, 902, 99
Impellizzeri, C. M. V., et al., 2019, ApJL, 884, L28
Izumi, T., Kawakatu, N., & Kohno, K., 2016a, ApJ, 827, 81
Izumi, T., et al., 2016b, MNRAS, 459, 3629
Izumi, T., et al., 2018, ApJ, 867, 48
Kawaguchi, T., et al., 2018, PASJ, 70, 93
King, A. & Pounds, K., 2015, ARA&A, 53, 115
Kormendy, J. & Ho, L. C., 2013, ARA&A, 51, 511
Lutz, D., et al., 2020, A&A, 633, A134
Maloney, P. R., Hollenbach, D. J., & Tielens, A. G. G. M., 1996, ApJ,
466, 561
Namekata, D., & Umemura, M., 2016, MNRAS, 460, 980
Muller-Sanchez, F., et al., 2011, ApJ, 739, 69
Richings, A. J., & Faucher-Giguère, C.-A., 2018, MNRAS, 474, 3673
Scannapieco, E., & Brügen, M., 2015, ApJ, 805, 158
Schneider, E. E., & Robertson, B. E., 2017, ApJ, 834, 144
Scoville, N., & Murchikova, L., 2013, ApJ, 779, 75
Tsuboi, M., et al., 2017, ApJ, 842, 94
Veilleux, S., et al., 2020, A&ARv, 28, 2
Wada, K., Schartmann, M., & Meijerink, R., 2016, ApJL, 828, L19
Wada, K., Yonekura, K., & Nagao, T., 2018, ApJ, 867, 49
Walmsley, C. M., 1990, A&AS, 82, 201

5