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We performed a systematic analysis of the 4.67 $\mu$m CO ro-vibrationalabsorption band toward nearby active galactic nuclei (AGNs) and analyzed theabsorption profiles of ten nearby galaxies collected from the AKARI and Spitzerspectroscopic observations that show the CO absorption feature by fitting aplane-parallel local thermal equilibrium gas model. We found that CO gas iswarm (200--500 K) and has a large column density($N_\mathrm{H}\gtrsim10^{23}~\mathrm{cm^{-2}}$). The heating of the gas is notexplicable by either UV heating or shock heating because these processes cannotrepresent the large column densities of the warm gas. Instead, X-ray photonsfrom 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 powersource. The hydrogen column density estimated from the CO band is smaller thanthat inferred from X-ray observations. These results indicate that the regionprobed by the near-infrared CO absorption is in the vicinity of the nuclei andis located outside the X-ray emitting region. Furthermore, the covering factorsclose to unity required by the observed deep absorption profiles suggest thatthe probed region is close to the continuum source, which can be designated asthe inner rim of the obscuring material around the AGN.

The Near-Infrared CO Absorption Band as a Probe to the Innermost Part of an AGN Obscuring Material