Understanding the mass growth of supermassive black holes in the universe
Masatoshi Imanishi 1,2
1

National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan2
Department of Astronomy, School of Science, The Graduate University for Advanced Studies, SOKENDAI, Mitaka, Tokyo 181-8588, Japanmasa.imanishi@nao.ac.jp

Abstract
We propose ngVLA high-angular-resolution (∼1 mas) dense molecular line observations at ∼3 mm (∼85 GHz)of nearby active galactic nuclei (AGNs) which are believed to contain actively mass-accreting supermassive black
holes (SMBHs). We aim to scrutinize the morphological and dynamical properties of the surrounding <10-pc-scalematerial, the so-called dusty molecular torus, with <∼1 pc spatial resolution. Our scientific goals are to unravel thenature of this mass reservoir just outside the central mass-accreting SMBHs and to understand how to fuel the central
SMBHs by removing angular momentum and achieve SMBH mass growth in the universe.
Key words: quasars: supermassive black holes — galaxies: nuclei — galaxies: Seyfert

1. Introduction : Role of “Torus” in Active Galactic
Nuclei
Active galactic nuclei (AGNs) are galaxy nuclei that shine
very brightly from compact regions. The bright compact emission is believed to originate in an actively mass-accretingsupermassive black hole (SMBH) with mass of >106 M⊙ .
Optical spectroscopy of AGNs has found that some display−1
broad (>
∼2000 km s ) emission lines (classified as type-1),−1while others show only narrow (<∼1000 km s ) emission lines(type-2). It is now widely believed that the presence of type1 and -2 AGNs can naturally be explained by the presenceof <∼10 parsec (pc)-scale toroidally distributed dust and gas,
the so-called dusty molecular “torus”, which surrounds a UVoptical-continuum-emitting accretion disk around an SMBH
and the sub-pc-scale broad-line-emitting regions photo-ionizedby the continuum emission (Antonucci 1993; Honig 2019) (seeFigure 1).
The torus is a structure where a large amount of dense material is present in a concentrated form just outside the massaccreting SMBH, and so is believed to be an important mass
reservoir to fuel the central SMBH. However, according to
the classical torus picture, the torus is thought to be less massive than the central SMBH and rotate with almost Keplerianmotion, largely affected by the gravity of the central massdominating SMBH, because molecular viscosity in the torus
is too small to significantly extract the angular momentum ina short timescale (Wada et al. 2009; Wada et al. 2016). In thiscase, mass in the torus will not be efficiently transported inward
and no significant mass-accretion onto the SMBH will happen.
This means that the SMBH will not be ignited and no luminous AGN will be observed in the universe. However, there
are many luminous AGNs found in the universe. There must
be still unknown physical mechanisms that realize mass fueling from the torus to the central SMBH by removing angularmomentum. It is of crucial importance to observationally unravel the secret mechanisms if we are to fully understand how
luminous AGNs appear and how SMBHs grow in mass in our
universe.

Fig. 1. The widely accepted unified model of AGNs. The mass-accreting SMBH consists of a central SMBH and an accretion disk (<0.1
pc scale), both of which are thought to be surrounded by toroidally
distributed dust and dense molecular gas, the so-called “torus” with
<10 pc scale. Fast-moving gas clouds in the vicinity of the central∼
SMBH, dominated by the strong SMBH’s gravity, are photo-ionized
by UV-optical radiation from the accretion disk, forming the so-called
broad line emitting regions (BLRs). When we see the BLRs directly,
from the face-on direction of the torus, the AGN is observed as type-1
which displays broad (FWHM > 2000 km s−1 ) optical emission lines.
When we see the torus from an edge-on direction and optical emission
from the BLRs is blocked by the torus, the AGN is observed as type-2
which does not show broad optical emission lines.

2. Case for NGC 1068 - the Nearby Archetypal AGN
The torus-based AGN unification paradigm was originallyproposed from observations of the nearby type-2 AGN, NGC
1068 (z ∼ 0.0038, distance ∼ 14 Mpc, 1 arcsec is ∼70 pc)(Antonucci & Miller 1985). Observationally scrutinizing theproperties of the putative dusty molecular torus in NGC 1068
is an important first step toward understanding what the torus
is and what its role is in AGNs. However, the apparent size′′of the putative <∼10 pc-scale torus is small (e.g., 10 pc is 0. 15
at the distance of NGC 1068), so high-spatial-resolution observations are indispensable to spatially resolving it clearly. The

2[Vol. ,

use of ALMA has started to bring about significant advancesin our understanding of the compact dusty molecular torus, be′′cause high-angular-resolution (<∼0. 1) and high sensitivity data
can now routinely be obtained at the (sub)millimeter wavelength, where rotational (J) transition lines of many abundant
molecules exist.
In the case of NGC 1068, radio jet emission and >∼10 pcscale optical [OIII] line emission photo-ionized by the AGN
radiation extend to the north-south direction in the vicinityof the mass-accreting SMBH (Evans et al. 1991; Das et al.
2006; Gallimore et al. 2004). Since such emission can easily escape along a direction almost perpendicular to the putative dusty molecular torus, the rotating torus in NGC 1068
is presumed to be located roughly along the east-west direction of the SMBH. Imanishi et al. (2018) conducted ALMA
∼0.′′ 04–0.′′ 07 (2.8 pc × 4.9 pc)-resolution observations of the
NGC 1068 nucleus using dense molecular tracers, HCN and
HCO+ J=3–2 lines, and revealed almost east-west-orienteddense molecular emission both morphologically and dynamically (Figure 2). This observational result conformed to what
people had expected for the torus in NGC 1068. The dense
molecular mass in the NGC 1068 torus was estimated to be
Mtorus ∼ 105−6 M⊙ (Imanishi et al. 2016; Garcia-Burillo et al.
2016; Garcia-Burillo et al. 2019), which is much smaller thanthe central SMBH mass with MSMBH ∼ 1×107 M⊙ (Greenhill
et al. 1996).

Fig. 2. (Left) Integrated intensity (moment 0) map and (Right) intensity-weighted mean velocity (moment 1) map of the HCN J=3–2 emission line (= a dense molecular gas tracer) from the putative torus of
NGC 1068. The emission is elongated along the almost east-west direction as expected (Left). The blueshifted and redshifted motions arealso aligned to the same east-west direction (Right). Adopted from
Imanishi et al. (2018).

However, it was also found that (1) unlike the predictionof the classical torus model, the velocity difference between
the blueshifted and redshifted components at the probed physical scale (∼3 pc) is much smaller than that expected from
Keplerian rotation dominated by the central SMBH, and (2)the dense molecular emission is blueshifted (redshifted) at thewestern (eastern) part of the torus, while both (a) innermost(<∼1 pc) ∼22 GHz H2 O maser emission and (b) dense molecular emission in the host galaxy (50–150 pc scale) outside thetorus, along the torus direction, show redshifted (blueshifted)
components at the western (eastern) part (Greenhill et al.
1996; Gallimore et al. 2004; Imanishi et al. 2018). Namely,
dense molecular gas in the putative torus looks to be counterrotating with respect to the innermost (<∼1 pc) H2 O maser
emission and outer (50–150 pc) host galaxy dense moleculargas. Figure 3 illustrates the observed complex dynamics ofdense molecular gas at the NGC 1068 nucleus. Very surprising
torus properties were revealed in NGC 1068.

Fig. 3. Observed dynamical properties of dense molecular and H2 O
maser emission at the inner part of NGC 1068.
(Top Left):Intensity-weighted mean velocity (moment 1) map of HCN J=3–2
emission line in the host galaxy (50–150 pc) scale (Imanishi et al.
2018). The thin solid line crossing the torus at the center indicates
the torus axis (PA ∼ 105◦ ). (Top Right): Moment 1 map of HCN
J=3–2 in the torus (1–5 pc) scale (Imanishi et al. 2018). (Bottom): H2 O
maser emission dynamics at the innermost (<∼1 pc) part. Modified from
Gallimore et al. (2004). Velocity relative to their adopted systemic velocity is color coded. The coordinates are offset (in mas) from the
VLBA 5 GHz continuum shown as contours. North is up and east isto the left in all plots. The length of the thick horizontal bar corresponds to 140 pc, 5 pc, 1.4 pc in the top-left, top-right, bottom panel,
respectively. Thick curved blue and red arrows indicate blueshifted andredshifted motion relative to the systemic velocity of NGC 1068, respectively. The western side is redshifted for maser emission at <∼1 pc(bottom) and dense molecular emission in the host galaxy at 50–150 pc
scale (top left), but is blueshifted for the torus dense molecular emission
at 1–5 pc scale (top right).

Later, ALMA ∼0.′′ 02 (∼1.4 pc)-resolution HCN J=3–2 line
data of the NGC 1068 torus unveiled the presence of inner (<∼2pc) and outer (>2
pc)
dynamicallydecoupleddensemolecu∼lar gas components, which are counter-rotating to each other(Figure 4) (Impellizzeri et al. 2019; Imanishi et al. 2020). Suchcounter-rotating molecular gas can easily remove the angularmomentum of the torus, transport torus material inward, andnaturally explain the observational fact that NGC 1068 is observed as a luminous AGN (LAGN > 4 × 1044 erg s−1 ) by accreting a sufficient amount of mass onto the central SMBH. We
hypothesize that a compact massive gas clump fell into the preexisting torus and altered the rotation of the outer torus (Figure
5).
In this way, our ALMA dense molecular line observational
results have clearly demonstrated the power of high-spatialresolution (∼1 pc) to unravel the interesting dynamic nature of
the compact (<∼10 pc) torus in this particular nearby archetypal
AGN, NGC 1068. We have obtained an important clue to understand how to fuel the central SMBH by transporting torus
material inward and ignite AGN activity.

No. ]

Fig. 4. Moment 1 map of the HCN J=3–2 emission of the torus in NGC
1068, revealed from ALMA very high-angular-resolution (∼0.′′ 02 or∼1.4 pc) data (Imanishi et al. 2020). The inner dense molecular gas displays redshifted (blueshifted) motion at the western (eastern) part in asimilar way to the innermost (<∼1 pc) H2 O maser emission (Greenhill etal. 1996; Gallimore et al. 2004), while the outer one shows blueshifted(redshifted) motion at the western (eastern) side, as previously observed in ∼0.′′ 04–0.′′ 07-resolution HCN and HCO+ J=3–2 line data
(Imanishi et al. 2020).

3
ngVLA is also about an order of magnitude larger than ALMAat ∼3 mm (∼85 GHz). Both the high-angular-resolution andhigh sensitivity of ngVLA will enable us to reveal the detailed dynamical properties of torus dense molecular gas with
<0.1 pc physical scale in very nearby (∼15 Mpc) AGNs,∼
such as NGC 1068. The physical origin of the angular momentum removal in torus dense molecular gas will be better understood. For example, (1) do instability and/or shocks
happen at the border of the counter-rotating dense molecular
gas in the NGC 1068 torus, which dramatically extract angular momentum there? (2) Do star-formation happen in thetorus which also helps significantly remove angular momentum
and enhance mass-accretion inward (Wada & Norman 2002)?
Observations of bright shock tracer lines (e.g., SiO, CH3 OH,
SO, SO2 ) and/or star-formation indicators (e.g., hydrogen recombination lines such as H40α or H42α; Bendo et al. (2015))
in the 3mm band using ngVLA will provide invaluable information for these kinds of activity in the torus in a completely
dust-extinction-free manner.
Most importantly, by utilizing an order of magnitude better
angular resolution and high sensitivity of ngVLA, we can extend the interesting case study of NGC 1068 to other luminous
AGNs and non-AGNs at further distance (>15 Mpc). This is
crucial to test the proposed grand-unified picture, by observing
a statistically significant number of luminous AGNs and nonAGNs at <∼1 pc spatial resolution. Using ngVLA, we aim toanswer the fundamental questions of “How does AGN activity happen?” and “How do SMBHs in the universe grow in
mass?”.
References

Fig. 5. Proposed scenario for the NGC 1068 torus: a massive compact
gas clump collided with the western torus (right side) from a far side
and altered the rotation there from redshifted to blueshifted motion,
except for the innermost almost-Keplerian rotating molecular gas component whose dynamics is strongly governed by the central SMBH’s
gravity. Blueshifted and redshifted motions are shown with blue andred arrows, respectively. This scenario can naturally explain all observational results presented by Imanishi et al. (2020), including larger
turbulence in the western torus and rotation of outer torus which ismuch slower than Keplerian.

3. Prospect for ngVLA
Although this is still only one example for NGC 1068, it
is natural to propose the grand-unified picture for the origin
of AGN phenomena in the universe. (1) Luminous AGNs are
the population where the angular momentum of torus dense
molecular gas is largely removed. (2) Non-AGNs do not havedense molecular torus to be fueled to the central SMBHs, or
do have dense molecular torus which is dynamically rotationdominated and angular momentum is not sufficiently removed.
The ngVLA will realize ∼1 mas angular resolution in the
3mm wavelength range, where bright dense molecular lines(e.g., HCN and HCO+ J=1–0) exist. Using ngVLA, we canachieve nearly an order of higher angular resolution for dense
molecular line observations than ALMA. The collecting area of

Antonucci, R. R. J. 1993, ARA&A, 31, 473
Antonucci, R. R. J. & Miller, J. S. 1985, ApJ, 297, 621
Bendo, G. J., Beswick, R. J., D’Cruze, M. J., et al. 2015, MNRAS,
450, L80
Das, V., Crenshaw, D. M., Kraemer, S. B., & Deo, R. P. 2006, AJ, 132,620
Evans, I. N., Ford, H. C., Kinney, A. L. et al. 1991, ApJL, 369, L27
Gallimore, J. F., Baum, S. A., & O’dea, C. P., 2004, ApJ, 613, 794
Garcia-Burillo, S., Combes, F., Ramos Almeida, C., et al. 2016, ApJL,
823, L12
Garcia-Burillo, S., Combes, F., Ramos Almeida, C., et al. 2019, A&A,
632, A61
Greenhill, L. J., Gwinn, C. R., Antonucci, R., & Barvainis, R. 1996,
ApJ, 472, L21
Honig, S. F. 2019, ApJ, 884, 171
Imanishi, M., Nakanishi, K., & Izumi, T. 2016, ApJL, 822, L10
Imanishi, M., Nakanishi, K., Izumi, T., & Wada, K. 2018, ApJL, 853,
L25
Imanishi, M., Nguyen, D. K., Wada, K., et al. 2020, ApJ, 902, 99
Impellizzeri, C. M. V., Gallimore, J. F., Baum, S. A., et al. 2019,
ApJL, 884, L28
Wada, K., & Norman, C. A. 2002, ApJL, 566, L21
Wada, K., Papadopoulos, P. P., & Spaans, M. 2009, ApJ, 702, 63
Wada, K., Schartmann, M., & Meijerink, R. 2016, ApJL, 828, L19