X-ray and CO-Derived Column Densities in AGN: A Study of Obscuration Properties in CTAGN and Non-CTAGN

Abstract

Obscuration in active galactic nuclei (AGN) provides insights into the material surrounding the central engine. Compton-thick AGN (CTAGN), characterized by a column density of $N_{\mathrm{H}}\ge 1.5\times 10^{24}{\mathrm{cm}}^{-2}$, are heavily obscured by dust and gas. While X-ray observations primarily determine this column density, the sub-mm obscuration properties of CTAGN remain less explored. We analyze archival ALMA CO(3-2) data for CTAGN and non-CTAGN from the 70-month $\text{Swift}$/BAT catalog and other X-ray surveys. Integrated intensity maps (moment 0) reveal dense gas concentrated around the nucleus. Assuming a constant CO-to-${\mathrm{H}}_2$ conversion factor, $X_{\mathrm{CO}}=2.2\times 10^{20}{\mathrm{cm}}^{-2}({\mathrm{K}\mathrm{km}\mathrm{s}}^{-1}{)}^{-1}$, we find that molecular hydrogen column densities ($N_{{\mathrm{H}}_2}$) are generally lower than X-ray-derived total hydrogen column densities ($N_{\mathrm{H}}$). However, $N_{{\mathrm{H}}_2}$ values in this work are slightly higher than in previous studies due to the adopted conversion factor. The discrepancy between $N_{\mathrm{H}}$ and $N_{{\mathrm{H}}_2}$ aligns with prior findings that X-ray-derived values tend to be higher, except for non-CTAGN, where $N_{{\mathrm{H}}_2}$ can exceed $N_{\mathrm{H}}$. Kendall and Spearman tests indicate a positive monotonic correlation, though not statistically significant, suggesting a complex interplay of factors. The optically thick nature of CO in dense regions may contribute to the observed differences. Our results highlight the need for an accurate CO-to-${\mathrm{H}}_2$ conversion factor in deriving column densities, potentially offering an alternative method for identifying CTAGN. Future studies with larger datasets and refined methodologies are essential for a deeper understanding of sub-mm and X-ray properties in AGN.

This paper has been accepted for publication in PASA

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