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

We explore millimeter line diagnostics of an obscuring molecular torus modeled by a hydrodynamic simulation with three-dimensional non-local thermodynamic equilibrium (nonLTE) radiative transfer calculations. Based on the results of high-resolution hydrodynamic simulation of the molecular torus around an active galactic nucleus (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. We find that similar transition coefficients for HCN and HCO rotational lines lead to a natural concordance of the level population distributions of these molecules and the line ratio RHCN/HCO+ . 1 for the same molecular abundance value over two orders of magnitude. Our results suggest that HCN must be much more abundant than HCO (yHCN & 10× yHCO+) in order to obtain a high ratio (RHCN/HCO+ ∼ 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