The Structure and Dynamics of an AGN Torus: CO Line Predictions for ALMA from 3D Hydrodynamical Simulations with X-ray Driven Chemistry

TL;DR

This paper models the chemical and dynamical structure of AGN tori using 3D hydrodynamical simulations with X-ray chemistry, predicting CO line emissions to aid ALMA observations of AGNs.

Contribution

It introduces a detailed 3D hydrodynamic and chemical model of AGN tori, including X-ray driven chemistry and radiative transfer, to predict molecular line emissions.

Findings

Mid-J CO lines are effective probes of density and dynamics in AGN centers.

Higher-J CO lines are suitable for gas mass estimation in AGN tori.

[C II] 158 um emission traces hot central regions and is useful for high-redshift AGN observations.

Abstract

Many efforts have been made to model the mass distribution and dynamical evolution of the circumnuclear gas in active galactic nuclei (AGNs). However, chemical evolution is not included in detail in three-dimensional (3-D) hydrodynamic simulations. The X-ray radiation from the AGN can drive the gas chemistry and affect the thermodynamics, as well as the excitation of the interstellar medium (ISM). Therefore, we estimate the effects (on chemical abundances and excitation) of X-ray irradiation by the AGN, for atomic and molecular gas in a 3-D hydrodynamic model of an AGN torus. We obtain the abundances of various species from an X-ray chemical model. A 3-D radiative transfer code estimates the level populations, which result in line intensity maps. Predictions for the CO J=1-0 to J=9-8 lines indicate that mid-J CO lines are excellent probes of density and dynamics in the central (<60 pc) region of the AGN, in contrast to the low-J CO lines. Analysis of the X_CO/\alpha conversion factors shows that only the higher-J CO lines can be used for gas mass determination in AGN tori. The [C II] 158 um emission traces mostly the hot (T_k>1000m K) central (<60 pc) region of the AGN torus. The [C II] 158 um line will be useful for ALMA observations of high redshift (z>1) AGNs. The spatial scales (>0.25 pc) probed with our simulations match the size of the structures that ALMA will resolve in nearby (<45 Mpc at 0.01") galaxies.