HCN to HCO+ millimeter line diagnostics of AGN molecular tori.: 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 radiative transfer calculations. Based on the results of a high-resolution hydrodynamic simulation of the molecular torus around an active galactic nucleus, we calculate the intensities of the HCN and HCO+ rotational lines as two representative high-density tracers. 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 the HCN and HCO+ rotational lines lead to a natural concordance of the level population distributions of these molecules and a line ratio RHCN/HCO+ less than or similar to 1 for the same molecular abundance value over 2 orders of magnitude. Our results suggest that HCN must be much more abundant than HCO+ (y(HCN) greater than or similar to 10y(HCO+)) in order to obtain the high ratio (RHCN/HCO+ similar to 2) observed in some nearby galaxies. There is a remarkable dispersion in the relation between integrated intensity and column density, indicative of possible shortcomings of the HCN(1-0) and HCO+(1-0) lines as high-density tracers. The internal structures of inhomogeneous molecular tori down to subparsec scales in external galaxies will be revealed by the forthcoming Atacama Large Millimeter/submillimeter Array. Three-dimensional radiative transfer calculations of molecular lines with a 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.