Molecular Gas Disk Structures around AGNs

TL;DR

This paper presents high-resolution simulations of the molecular gas structures around supermassive black holes, revealing a clumpy, turbulent disk with properties consistent with recent infrared observations.

Contribution

It introduces a novel simulation including molecular hydrogen formation in the nuclear region, providing detailed insights into the morphology and kinematics of circumnuclear molecular gas.

Findings

Molecular hydrogen forms a thin disk within 5 pc of the nucleus.

Gas column densities vary widely due to clumpiness, with high densities observed at certain viewing angles.

The simulated gas structures match recent infrared observations.

Abstract

We present new high resolution numerical simulations of the ISM in a central R ~32 parsecs region around a supermassive black hole at a galactic center. Three-dimensional hydrodynamic modeling of the ISM (Wada & Norman 2002) with the nuclear starburst now includes tracking of the formation of molecular hydrogen out of the neutral hydrogen phase. In a quasi equilibrium state, mass fraction of H2 is about 0.4 (total H2 mass is ~1.5 10^6 Msun) of the total gas mass for the uniform far UV (FUV) with G_0 = 10. The gas forms an inhomogeneous disk, whose scale-height becomes larger in the outer region. H2 forms a thin nuclear disk in the inner ~ 5 pc, which is surrounded by molecular clouds swelled up toward h < 10 pc. The velocity field of the disk is highly turbulent in the torus region, whose velocity dispersion is ~ 20 km/s on average. Average supernova rate (SNR) of ~ 5 10^-5/yr is large enough to energize these structures. Gas column densities toward the nucleus larger than 10^22 cm^-2 are observed if the viewing angle is smaller than \theta_v ~ 50 deg from the edge-on. However, the column densities are distributed over almost two orders of magnitude around the average for any given viewing angle due to the clumpy nature of the torus. For a stronger FUV (G_0 =100), the total H2 mass in an equillibrium is only slightly smaller (~ 0.35). Finally the morphology and kinematics of the circumnuclear molecular gas disks emerging from our models is similar to that revealed by recent near infrared observations using VLTI/Keck.