Identifying Compton-thick active galactic nuclei in the COSMOS. II. Searching among mid-infrared selected AGNs

TL;DR

This study identifies heavily obscured Compton-thick AGNs in the COSMOS survey using mid-infrared selection and X-ray stacking, revealing a smaller fraction than predicted and highlighting the challenge of detecting these hidden sources.

Contribution

It introduces a method combining MIR diagnostics and X-ray stacking to identify CT-AGNs among X-ray undetected sources, finding a lower-than-expected population.

Findings

7 to 23 CT-AGN candidates identified via MIR diagnostics.

Stacked X-ray analysis confirms absorption by Compton-thick material.

Only 2.1% of MIR-selected AGNs are CT-AGNs, below model predictions.

Abstract

Compton-thick active galactic nuclei (CT-AGNs), defined by column density $\mathrm{N_H} \geqslant 1.5 \times 10^{24} \ \mathrm{cm}^{-2}$, are so heavily absorbed that their X-ray emission is often feeble, even undetectable by X-ray instruments. Nevertheless, their radiation is expected to be a substantial contributor to the cosmic X-ray background (CXB), predicting that CT-AGNs should comprise at least $\sim$30% of the total AGN population. In the Cosmological Evolution Survey (COSMOS), the identified CT-AGN fraction falls far below theoretical expectations, indicating that a substantial population of CT-AGNs is hidden due to their low photon counts or their flux below the current flux limits of X-ray instruments. This work focuses on identifying CT-AGNs hidden in mid-infrared (MIR)-selected AGNs. First, we selected a sample of 1,104 MIR-selected AGNs that were covered but individually undetected by X-ray. Next, we reduced the X-ray data in the COSMOS and analyzed multiwavelength data in our sample to derive the key physical parameters required for CT-AGN identification. Using MIR diagnostics, we first find out 7 to 23 CT-AGN candidates. Their subsequent X-ray stacking analysis reveals a clear detection at $>3\sigma$ significance in the soft band and only $>1\sigma$ significance in the hard band. We fit the stacked soft- and hard-band fluxes with a physical model and confirm that these sources are absorbed by Compton-thick material. However, CT-AGNs constitute only 2.1% (23/1104) of our sample, significantly below the fraction predicted by CXB synthesis models, indicating that a considerable population of CT-AGNs remains missed by our selection. A comparison of host-galaxy properties between CT-AGNs and non-CT-AGNs reveals no significant differences.