High resolution imaging of the molecular torus in NGC 1052 with VLBISatoko Sawada-Satoh1, Do-Young Byun2, Sang-Sung Lee2, Se-Jin Oh2, Duk-Gyoo Roh2, Seiji Kameno3,Jae-Hwan Yeom2, Dong-Kyu Jung2, Chungsik Oh2, Hyo-Ryoung Kim2, and Ju-Yeon Hwang2

1: Yamaguchi University, Japan, 2: Korea Astronomy and Space Science Institute, Korea, 3: Joint ALMA Observatory, Chile(Sawada-Satoh et al. 2016, ApJL 830 L3; Sawada-Satoh et al. 2019, ApJL 872 L21)

Abstract

HCN absorption

We present the first subparsec-scale imaging of HCN J=1-0 and HCO+ J=1-0 absorptions in
the circumnuclear region of the nearby radio galaxy NGC 1052. Our 3-mm VLBIobservations with the Korean VLBI Network have spatially resolved those absorptions
against a double-sided nuclear jet structure. The absorption features show high opticaldepth concentrated on the receding jet side, where the free-free absorption occurred due tothe circumnuclear torus. The size of the foreground absorbing molecular gas is estimated tobe on approximately one-parsec scales. These results are most naturally explained by acircumnuclear torus that consist of several phase layers, perpendicular to the jet axis ofNGC 1052. HCN and HCO+ absorbing gas could lie in the cooler molecular layer. The
redshifted velocities of the absorption features account for the infall motion onto the centralengine, such as an H2O maser emission.

Two velocity features of HCN absorption (I & II) are detected with KVN (Fig.3), which arealso found in the absorption profile from the IRAM PdBI data[5]. The absorption depths inthe KVN spectral profile are deeper than those of the PdBI (2–6%). KVN images resolve theHCN absorption spatially, and show that high HCN optical depth is localized on the westernreceding jet side (Fig.4). HCN absorbing gas could be associated with the cooler molecularlayer of the torus (Fig.5). The HCN absorbing gas is more likely to clumpy, rather than alarge homogeneous structure with a single velocity.

Introduction
It is generally accepted that pronounced activity of an active galactic nucleus (AGN) isdriven by the accretion of gas from its circumnuclear torus onto a supermassive black hole.
Molecular gas in the central regions of extragalaxies provides important clues to the nuclear
environment of AGNs. High angular resolution studies of molecular gas in the center of theexternal galaxies (<1kpc) have been obtained with millimeter and submillimeterinterferometers. The size of the torus, however, is smaller than 10pc, and a milliarcsecond(mas) resolution is required to study its internal structure in nearby AGNs. VLBI
observations have revealed the parsec- or subparsec-scale morphology of nearby AGNs.
Although thermal emission lines from molecular gas are not luminous enough to detect withthe VLBI, VLBI maps can display thermal absorption lines of the gas in silhouette against abright background synchrotron radiation source with a mas resolution. To investigate thegeometry and physical properties of the molecular gas in the circumnuclear region ofNGC1052, we have performed high-resolution observations toward HCN J=1–0 and HCO+
J=1-0 absorption lines with the Korean VLBI Network (KVN).

Fig.3 HCN absorption lines
detected with KVN

Fig.4 HCN optical depth maps (color), overlaid by a continuummap (contour)

About NGC 1052
NGC 1052 hosts a well-studied circumnuclear torus surrounding its central engine.
‣ A parsec-scale plasma torus, which obscure 0.1pc and 0.7pc of the eastern and western
jets, respectively[1].
‣ Positionally coincidence of H2O maser emission and a plasma-obscuring torus[2].
‣ A torus model of several phase layers is proposed; a hot plasma layer at the inner surface,a warm molecular gas layer where H2O maser arises, a cooler molecular layer[2,3].
‣ Gases of H2O maser emission and OH absorption are located where the plasma torus
obscure, which supports the torus model with several layer[2,4].
‣ The spectra of H2O maser and OH absorption are redshifted with respect to the systemic
velocity of the galaxy, and the fact implies the on-going material infall onto the supermassive black hole[2,4].
‣ HCN and HCO+ absorptions have been detected against an unresolved core with theIRAM PdBI, while it is unclear where they are associated with[5].

HCO+ absorption
Broad HCO+ absorption is detected in the velocity range from 1350 to 1850 km s-1, with theblueshifted part having more extended wing than the redshifted part. The HCO+ absorption
is spatially resolved on subparsec scales. Concentration of high HCO+ opacity on thewestern receding jet component strongly suggests that HCO+ absorption is associated withthe parsec-scale circumnuclear torus. The broad velocity width could be a complex of some
rapid motion with multiple clumpy gas clouds at various different velocities, such asturbulence, interaction, and so on.

Kameno+ (2005)(a)100-10(b)10Flux Density [mJy]

Flux Density [mJy]10

Flux Density [mJy]

Sawada-Satoh+ (2008)

Fig.5 A possible torus model

0-10Vsys-201200

Vsys
1200

Radial Velocity [km s-1]

2000

1200

10(f)

Vsys

010-2

642

1600

Radial Velocity [km s-1]

2000

(e)

0-100-2
RA offset [mas]

-4

Vsys

-20-61200

1600

Radial Velocity [km s-1]

2000

Fig.7 Spectra of HCO+ absorption at different locations

Fig.6 HCO+ spectral profile
0.201200-402000(d)-200.2 pc

Fig.2 A torus model with several layers

1600

Radial Velocity [km s-1]

-10Flux Density [mJy]

DEC offset [mas]2

Fig.1 Double-sided radio jet of NGC 1052

Vsys-20

1600

Flux Density [mJy]4

Radial Velocity [km s-1]

2000

0-10-20

1600

(c)

0.4

00.2

0.4

0

0.2

0.4

1214

1321

1429

1537

1644

1752

1859

1967

2074

2Observations

0To investigate distributions of HCN and HCO+ absorption in NGC 1052, KVN observations
were carried out at 3-mm band. The resultant beam size was 1.5 x 0.9 mas (0.14 x 0.09 pc).

HCO+ J=1-0

5 March 201517 June 2017

Bandwidth [MHz]

128

512

Rest frequency [GHz]89.08889.188References
1. Kameno, S., et al. 2001, PASJ, 53, 169
2. Sawada-Satoh, S., et al. 2008, ApJ, 680, 191
3. Kameno, S., et al. 2005, ApJ, 620, 145
4. Impellizzeri, V., et al. 2008, Proc. 9th EVN Symposium, POS(IX EVN Symposium), 3
5. Liszt, H., & Lucas, R. 2004, A&A, 428, 445

DEC Offset [mas]Observation date

HCN J=1-0-220-220-2420-2-4420-2

RA Offset [mas]Fig.8 HCO+ opacity channel maps averaging every 108 km s-1-4