From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs -- Modelling the molecular emission of a parsec-scale torus as found in NGC1068

TL;DR

This paper models the molecular emission of a parsec-scale torus in NGC1068, combining dynamical simulations, analytical turbulence models, and radiative transfer to interpret observations of gas kinematics and line fluxes.

Contribution

It introduces a comprehensive model integrating dynamics, turbulence, heating, and chemistry to explain molecular emission in AGN tori, calibrated with observations of NGC1068.

Findings

Gas kinematics explained by cloud collisions with a pre-existing ring.

Line fluxes reproduced within a factor of two using dense cloud models.

Turbulent mechanical energy is the main heating mechanism in thick gas disks.

Abstract

The accretion rates needed to fuel the central black hole in a galaxy can be achieved via viscous torques in thick disks and rings, which can be resolved by millimetre interferometry within the inner ~20pc of the active galaxy NGC1068 at comparable scales and sensitivity to single dish observations of the Circumnuclear Disk (CND) in the Galactic Center. To interpret observations of these regions and determine the physical properties of their gas distribution, we present a modelling effort that includes (i) a simple dynamical simulations involving partially inelastic collisions between disk gas clouds, (ii) an analytical model of a turbulent clumpy gas disk calibrated by the dynamical model and observations, (iii) local turbulent and cosmic ray gas heating and cooling via H2O, H2, and CO emission, and (iv) determination of the molecular abundances. We also consider photodissociation regions (PDR) where gas is directly illuminated by the central engine. We compare the resulting model datacubes of the CO, HCN, HCO+, and CS brightness temperatures to available observations. In both cases the kinematics can be explained by one or two clouds colliding with a pre-existing ring, in a prograde sense for the CND and retrograde for NGC1068. And, with only dense disk clouds, the line fluxes can be reproduced to within a factor of about two. To avoid self-absorption of the intercloud medium, turbulent heating at the largest scales, comparable to the disk height, has to be decreased by a factor of 50-200. Our models indicate that turbulent mechanical energy input is the dominant gas heating mechanism within the thick gas disks. In N1068, while the bulk of the AGN X-ray radiation is absorbed in a layer of Compton-thick gas inside the dust sublimation radius, the optical/UV radiation may enhance the molecular line emission from photodissociation regions by ~50% at the inner edge of the gas ring.