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

TL;DR

This study uses the 4.67 μm CO absorption band to probe the innermost regions of AGN obscuring material, revealing warm, dense gas likely heated by X-rays near the nucleus.

Contribution

It provides the first systematic analysis of the CO ro-vibrational absorption in nearby AGNs, linking warm gas properties to X-ray heating and the inner structure of obscuring material.

Findings

CO gas is warm (200-500 K) with high column densities.

X-ray photons are the most plausible heating source.

The probed region is close to the nucleus and outside the X-ray emitting zone.

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 ($N_\mathrm{H}\gtrsim10^{23}~\mathrm{cm^{-2}}$). 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.