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

Abstract

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 ($N_{\mathrm{H}}\gtrsim 10^{23}\mathrm{c}{\mathrm{m}}^{-2}$). 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 $N_{\mathrm{H}}\sim 10^{24}\mathrm{c}{\mathrm{m}}^{-2}$, 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.

arXiv commentAccepted for publication in ApJ. Duplicate sub figures in Figure 1 have been replaced with the correct ones (v2)

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