The Near-Infrared CO Absorption Band as a Probe to the Innermost Part of
  an AGN Obscuring Material
  We performed a systematic analysis of the 4.67 $\mu$m CO ro-vibrational
absorption band toward nearby active galactic nuclei (AGNs) and analyzed the
absorption profiles of ten nearby galaxies collected from the AKARI and Spitzer
spectroscopic observations that show the CO absorption feature by fitting a
plane-parallel local thermal equilibrium gas model. We found that CO gas is
warm (200--500 K) and has a large column density
($N_\mathrm{H}\gtrsim10^{23}~\mathrm{cm^{-2}}$). The heating of the gas is not
explicable by either UV heating or shock heating because these processes cannot
represent the large column densities of the warm gas. Instead, X-ray photons
from the nuclei, which can produce large columns of warm gas with up to
$N_\mathrm{H}\sim10^{24}~\mathrm{cm^{-2}}$, are the most convincing power
source. The hydrogen column density estimated from the CO band is smaller than
that inferred from X-ray observations. These results indicate that the region
probed by the near-infrared CO absorption is in the vicinity of the nuclei and
is located outside the X-ray emitting region. Furthermore, the covering factors
close to unity required by the observed deep absorption profiles suggest that
the probed region is close to the continuum source, which can be designated as
the inner rim of the obscuring material around the AGN.
