Multiphase Gas Morphology and Dynamics in the Circumnuclear Region
Satoko Sawada-Satoh 1
1

Graduate School of Sciences and Technology for Innovation, Yamaguchi University,
1677-1 Yoshida, Yamaguchi 753–8512
sss@yamaguchi-u.ac.jp

Abstract
Multiphase gas morphology and dynamics in the circumnuclear region of galaxies provide important clues forunderstanding the mechanism how active galactic nuclei (AGN) are fueled and release large mounts of energy toaffect their environment. Circumnuclear kinematics has been traced by a mixture of different gas phases ranging
from cold molecular gas to neutral atomic gas to hot ionized gas. Despite a growing number of circumnuclear
gas studies, we still lack a clear consensus what trigger the gas inﬂowing toward a central supermassive blackhole (SMBH) and subsequent feedback to host galaxy. The next generation Very Large Array will provide further
insights into the mass accretion and feedback processes, optimizing its broad frequency coverage from 1.2 to116
GHz, wide range of angular resolution spanning from 1000 to 0.1 milliarcsecond, and extremely high sensitivity. In
addition, the combination of high angular observations with other facilities will offer new opportunities for ultimate
understanding the complex circumnuclear gas morphology and dynamics.
Key words: galaxies: active — galaxies: nuclei

1. Introduction
It is generally proposed that the highly energetic activity of
AGN is powered by mass accretion onto a SMBH (Antonucci
1993). Accretion mass toward the center of galaxies fuels
the SMBH, and the gravitational energy of accretion mass isturned into radiation, winds and jets. Those AGN mechanicalenergy releases can play a signiﬁcant role in the evolution of
its host galaxy (e.g., Croton et al. 2006; Sijacki et al. 2007).
Morphology and dynamics of the circumnuclear gas in galaxies can set some constraints on the processes of AGN fuelingand feedback.
Mass accretion in active galaxies that host gas-rich circumnuclear environment is considered to be triggered by circumnuclear star formation and supernova explosions (e.g., Kawakatu& Wada 2008; Izumi et al. 2016). On the other hand, it remains
unclear whether such a process is also working properly in radio galaxies where circumnuclear star formation is inactive.
2. Dynamics on handreds-parsec scales
Interferometric observations towards nearby AGNs on
handreds-parsec scales have unveiled circumnuclear gas dynamics, including inﬂow, outﬂow and rotation in last three
decade. High angular resolution imagings of neutral atomic
hydrogen (H I) 21 cm line in Seyfert galaxies show that H I
absorbing gas could trace 100 parsec-scale rotation surrounding the center, which imply a circumnuclear disk (CND) (e.g.,
Gallimore et al. 1999). The H I lines can be observed at wider
velocity range, which could be associated with the inﬂow andoutﬂow motions from the center of radio galaxies (e.g., van
Gorkom et al. 1989; Morganti et al. 1998). Those motions havebeen traced by a different type of physical phases not only ionized and atomic gas but also molecular gas. Multiphase gas
observations have often reported that outﬂowing/inﬂowing gas

and rotations gas can coexist on the scale of 10–100 parsec(e.g., Ott et al. 2001).
To illustrate what has been found in the multiphase gas dynamics, we show here the results for several example AGNs.
One well-studied example is NGC 1275, a bright nearby radio galaxy. Inverted radio continuum spectra on the nuclear
counterpart jet of this galaxy is explained as free-free absorption (FFA) by dense ionized gas which forms a disk structure(Vermeulen et al. 1994; Levinson et al. 1995). Velocity structure of molecular gas in the core of NGC 1275 also support the
hypothesis of a rotation disk (Inoue et al. 1996; Wilman et al.
2005). Recent molecular line observations with the Atacama
Large Millimeter/Submillimeter Array (ALMA) also suggest
the presence of rotating and outﬂowing gases within 100 parsec in NGC 1275 (Nagai et al. 2019).
In the case of the nearby Seyfert 2 galaxy NGC 1068,
the circumnuclear disk and outlow are probed by the various molecular emission lines such as CO, HCN, HCO+ , andCS (e.g., Garcı́a-Burillo et al. 2014). Multi-line observations
have shown the signiﬁcant difference in molecular line ratios or chemical condition between the CND and the galactic
disk (Viti et al. 2014). Further high angular resolution studies of multiple molecular lines with the Very Long Baseline
Array (VLBA) and the ALMA reveal complex gas dynamics,
including rotation, counter-rotation and outﬂow within central 100 parsec (e.g., Gallimore et al. 2004; Impellizzeri et al.
2019; Imanishi et al. 2020). Highly dynamical properties ofcircumnuclear multiphase gas is also presented in the nearest
Seyfert 2 galaxy Circinus, through the collaboration of observational and theoretical studies (Wada et al. 2018a; Izumi et al.
2018; Wada et al. 2018b).
In addition, the combination of H I interferometric data andhigh resolution optical images by the Hubble Space Telescope
has proposed a multiphase circumnuclear disk with atomic hydrogen gas at inner radius and dust at outer radius in the nearby2radio galaxy NGC 4261 (Ferrarese et al. 1996; van Langevelde
et al. 2000).
3. Dynamics on parsec/subparsec scales
One milliarcsecond angular resolution is required to unveil
the parsec/subparsec-scale gas morphology and dynamics inside the CND of nearby AGNs with up to ∼ 100 Mpc of distance. While conventional millimeter interferometer, even the
ALMA long baselines, did not achieve such a high angularresolution, very long baseline interferometry (VLBI) has offered one milliarcsecond resolution. Compared to the case ofmultiphase gas studies on handred-parsec scales, gas tracers inparsec/subparsec-scale region are not many, because thermal
emissions are not luminous enough to detect with the VLBI ingeneral. Thus, circumnuclear gas morphology and dynamics
on 0.1–10 parsec scales have investigated by absorption such as
FFA and H I absorption on the non-thermal synchrotron radiation or bright non-thermal maser emission from excited molecular gas.
Multi-frequency VLBI observations have revealed parsecscale ionized gas torus which obscures the core or innermost
counter jet of the radio galaxies (e.g., Taylor 1996; Jones &Wehrle 1997; Tingay & Murphy 2001). In some case of them,
the torus is also traced by H I absorption (e.g., Peck & Taylor
2001; Struve & Conway 2010). H I absorption can also trace
the inﬂow toward the central SMBH (e.g., Taylor et al. 1999).
Very luminous extragalactic H2 O masers associated with AGN
have interpreted as a possible signature of a accretion disk (e.g.,
Miyoshi et al. 1995; Herrnstein et al. 1999), radiative shocks
from the nuclear jet (e.g., Peck et al. 2003) or nuclear outﬂows(e.g., Greenhill et al. 2003).
There is almost no report about AGN with H2 O maser clouds
coexisting with ionized gas in a circumnuclear torus, except anearby radio galaxy NGC 1052. The results of multiphase gas
dynamics in NGC 1052 are summarized here. VLBI observations toward the center of NGC 1052 at multiple frequency
bands have revealed the presence of a dense circumnuclear
ionized torus which obscures the very center of the galaxy(Kameno et al. 2001; Kadler et al. 2004). Follow-up simultaneous multi-frequency VLBI observations indicate that the H2 O
maser clouds are distributed where the FFA opacity is high inthe circumnuclear torus (Sawada-Satoh et al. 2008). VLBA images of OH absorption (Impellizzeri et al. 2008) also indicate
a similar distribution to that of FFA and a similar velocity tothat of H2 O maser emission. Kameno et al. (2005) has proposed the idea that the circumnculear torus consists of ionized
and molecular gas, depending on the physical properties inside
the torus. The idea of the torus with coexistence of ionized andmolecular gas is improved as a torus model with multiphasegas layers: a hot ionized gas layer at the innermost edge where
FFA opacity is large, a warm molecular gas interlayer where
the H2 O maser emissions arise, and a cooler molecular gas
layer where HCN and HCO+ molecular clumps lie (Fig. 1),
from the Korean VLBI Network observations (Sawada-Satohet al. 2016; Sawada-Satoh et al. 2019). The redshifted velocity proﬁles of H2 O, OH, HCN and HCO+ lines with respect tothe systemic velocity of the galaxy are likely indicative of ongoing gas infall motion onto the SMBH. Furthermore, ALMA

[Vol. ,
observations have found that the unresolved continuum emission of NGC 1052 casts many molecular absorption features(CO, HCN, HCO+ , SO, SO2 , CS, CN and H2 O), which canarise in a molecular torus surrounding the SMBH (Kameno etal. 2020).
The innermost radius of circumnuclear torus has been unclear, because the exact distance of FFA ionized gas from the
SMBH cannot be measured along the line of sight. The innermost ionized gas of the circumnuclear torus could be locatedclose to the broad line region (BLR), where clouds are ionizedby photons from the accretion disk around the SMBH. Recent
near-infrared Very Large Telescope Interferometer (VLTI) instrument, GRAVITY, have spatially resolved the BLR in abright quasar 3C273. The visibility phase is well ﬁtted by athick disk model in circular orbit smaller than 0.1 parsec witha rotation axis aligned with the radio jet direction of 3C273(Gravity Collaboration et al. 2018). The structure of BLR ispossibly a continuation of the parsec-scale circumnuclear torus
represented by FFA.
4. Impact of ngVLAThe next generation Very Large Array (ngVLA) will have agreat impact on studies of multiphase gas surrounding the circumnuclear region. It is designed to achieve various angularresolutions ranging from ∼ 1000 to 0.1 milliarcsecond and superb sensitivity of µJy level simultaneously, utilizing its threefundamental subarrays and additional long baselines. By ﬁlling the baseline gap between current interferometer and VLBI,
the ngVLA will allow us to explore the mass accretion processof nearby AGNs at various spatial scales from galactic disk,
CND to circumnuclear torus.
To fully understand the accretion and feedback of the AGN,
high angular resolution observations of multiple gas tracers arenecessary to probe each phase of the gas in the circumnuclear
region. The wide frequency coverage from 1.2 to 116 GHz
bands of the ngVLA can include many important atomic andmolecular transitions, which are the key to study physics andchemistry of the interstellar medium (ISM) in galaxies. The
broad bandwidth of ngVLA (up to 20 GHz) will enable to carryout the multi-line observations effectively.
5. Synergies with other facilities
Multi-frequency observations with the ngVLA and ALMA
will be highly synergistic with exploration of the ISM condition. The higher frequency bands of ALMA can observe themid- and high-J molecular transitions, while the ngVLA willdetect low-J lines. The combination of the ngVLA and ALMA
observations will play an important role to constrain the physical condition including density and temperature in the circumnuclear ISM.
The coordinated observations of the ngVLA with existing VLBI networks will considerably broaden the science ofparsec-scale circumnuclear gas, providing very long baselines
with high sensitivities. Especially, the cooperation between the
ngVLA and Square Kilometer Array (SKA), will boost sensitivity extremely. As the ngVLA and SKA both will be capableof detecting H I line in the atomic phase, the joint observation

No. ]

3

(a)

(b)
0

0.1

0.2

0.3

XDR

4

DEC Offset [mas]1656km/s

Circumnuclear
torus

0.2pc

Ionized gas

2

Westernrecedingjet

0Easternapproachingjet
HCN clouds

Approaching jet-2

Receding jet

The line of sight

-4
6

4

2

0
-2
RA Offset [mas]-4-6

Fig. 1. (a) Subparsec-resolution image of the HCN optical depth at VLSR = 1656 km s−1 in the circumnuclear region of NGC 1052, overlaid bya contour map of double-sided radio continuum jet structure at 89 GHz. High optical depth is concentrated on the central nuclear and receding jet
components, while no signiﬁcant absorption is seen against the approaching jet. (b) Schematic picture for the intersection of the circumnuclear torus in
NGC 1052. The torus consists of multiphase gas layers, hot ionized gas region on the innermost surface, warm excited molecular gas region at intermediate
layer, and cooler molecular region at outer layer. Molecular layer is composed of gas clumps smaller than 0.1 parsec (See Sawada-Satoh et al. 2016).

of H I absorption using these arrays will be a powerful tool tomake visible H I gas clumps in the circumnuclear region.
Finally, next-generation optical and infrared interferometers,such as VLTI, will provide complementary information about
the photoionized gas in the vicinity of SMBH inside the circumnuclear torus.
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