Circumnuclear Multi-phase Gas in Circinus Galaxy IV: absorption owing to
  high-$J$ CO rotational transitions
  We studied the absorption features of CO lines against the continuum
originating from the heated dust in the obscuring tori around active galactic
nuclei (AGNs). We investigated the formation of absorption lines corresponding
to the CO rotational transitions using three-dimensional non-LTE line transfer
simulations considering the dust thermal emission. As in Papers I--III of this
series, we performed post-processed radiative transfer calculations using the
"radiation-driven fountain model" (wada2016}, which yields a geometrically
thick obscuring structure around the nucleus. This model is consistent with the
spectral energy distribution of the nearest type-2 Seyfert galaxy, the Circinus
galaxy. We found that the continuum-subtracted channel maps of $J = 4-3$ and
higher transitions show absorption regions along the disk mid-plane for an
edge-on viewing angle. The spectra consist of multiple absorption and emission
features, reflecting the internal inhomogeneous and turbulent structure of the
torus. The deepest absorption feature is caused by the gas on the near-side of
the torus between $r =10$ and 15 pc, which is located in front of the
AGN-heated dust inside $r \simeq 5$ pc. We also found that a spatial resolution
of 0.5--1.0 pc is necessary to resolve the absorption features. Moreover, the
inclination angle must be close to the edge-on angle (i.e., $\sim 85^\circ$) to
observe the absorption features. The findings of the present study imply that
combining our radiation-hydrodynamic model with high-resolution observations of
CO (7-6) by ALMA can provide new information about the internal structure of
the molecular tori in nearby AGNs.
