Circumnuclear Multi-phase Gas in Circinus Galaxy I: Non-LTE calculations
  of CO lines
  In this study, we investigate the line emissions from cold molecular gas
based on our previous "radiation-driven fountain model" (Wada 2016), 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 r < 16 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 J=6, 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 $\tau \ll 1$ and $\tau \gg 1$. The CO-to-H2
conversion factor (X_CO), which can be directly obtained from the results, is
not a constant and depends strongly on the integrated intensity, and it differs
from the fiducial value for local objects. X_CO exhibits a large dispersion of
more than one order of magnitude, reflecting the non-uniform internal structure
of a "torus." We also 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 ISM.
