Circumnuclear Multi-phase Gas in the Circinus Galaxy. I. Non-LTE Calculations of CO Lines

Abstract

Abstract In this study, we investigate the line emissions from cold molecular gas based on our previous “radiation-driven fountain model,” which reliably explains the spectral energy distribution of the nearest type 2 Seyfert galaxy, the Circinus galaxy. Using a snapshot of the best-fit radiation-hydrodynamic model for the central pc, in which non-equilibrium X-ray-dominated region chemistry is solved, we conduct post-processed non-local thermodynamic equilibrium radiation transfer simulations for the CO lines. We obtain a spectral line energy distribution with a peak around , and its distribution suggests that the lines are not thermalized. However, for a given line of sight, the optical depth distribution is highly non-uniform between and . The CO-to-H2 conversion factor ( ), which can be directly obtained from the results and is not a constant, depends strongly on the integrated intensity and differs from the fiducial value for local objects. exhibits a large dispersion of more than one order of magnitude, reflecting the non-uniform internal structure of a “torus.” In addition, we found that the physical conditions differ between grid cells on a scale of a few parsecs along the observed lines of sight; therefore, a specific observed line ratio does not necessarily represent a single physical state of the interstellar medium.