HCN to HCO^+ Millimeter Line Diagnostics of AGN Molecular Torus I : Radiative Transfer Modeling

TL;DR

This study uses advanced 3D radiative transfer modeling based on hydrodynamic simulations to analyze millimeter line diagnostics of AGN molecular tori, revealing complex excitation states and chemical abundance implications for high-density tracers.

Contribution

It introduces a novel combination of hydrodynamic simulations with 3D radiative transfer calculations to study molecular line diagnostics in AGN tori, highlighting the importance of chemical abundance and structural inhomogeneity.

Findings

HCN must be more abundant than HCO+ to match observed line ratios.

Significant dispersion exists between integrated intensity and column density.

High-resolution ALMA observations will reveal detailed internal structures.

Abstract

We explore millimeter line diagnostics of an obscuring molecular torus modeled by a hydrodynamic simulation with three-dimensional nonLTE radiative transfer calculations. Based on the results of high-resolution hydrodynamic simulation of the molecular torus around an AGN, we calculate intensities of HCN and HCO^{+} rotational lines as two representative high density tracers. The three-dimensional radiative transfer calculations shed light on a complicated excitation state in the inhomogeneous torus, even though a spatially uniform chemical structure is assumed. Our results suggest that HCN must be much more abundant than HCO^{+} in order to obtain a high ratio ($R_{HCN/HCO+}\sim 2$) observed in some of the nearby galaxies. There is a remarkable dispersion in the relation between integrated intensity and column density, indicative of possible shortcomings of HCN(1-0) and HCO^{+}(1-0) lines as high density tracers. The internal structures of the inhomogeneous molecular torus down to subparsec scale in external galaxies will be revealed by the forthcoming Atacama Large Millimeter/submillimeter Array (ALMA). The three-dimensional radiative transfer calculations of molecular lines with high-resolution hydrodynamic simulation prove to be a powerful tool to provide a physical basis for molecular line diagnostics of the central regions of external galaxies.