Identifying Compton-thick AGNs in the COSMOS

II. Searching among mid-infrared selected AGNs

¹ School of Physics and Astronomy, Anqing Normal University, Anqing 246133, China
² Institute of Astronomy and Astrophysics, Anqing Normal University, Anqing 246133, China
³ School of Astronomy and Space Science, Nanjing University, Nanjing, Jiangsu 210093, China
⁴ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China
⁵ College of Physics and Electronic Engineering, Qujing Normal University, Qujing 655011, China
⁶ School of Physical Science and Technology, Kunming University, Kunming 650214, China
⁷ School of Science, Langfang Normal University, Langfang 065000, China

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 October 2025
Accepted: 4 March 2026

Abstract

Context. Compton-thick active galactic nuclei (CT-AGNs), defined by a column density of N_H ≥ 1.5 × 10²⁴ cm⁻², are so heavily absorbed that their X-ray emission is often feeble and can even be undetectable by X-ray instruments in some cases. Nevertheless, their radiation is expected to be a substantial contributor to the cosmic X-ray background (CXB), predicting that CT-AGNs would comprise at least ∼30% of the total AGN population.

Aims. Cosmological Evolution Survey (COSMOS) reported that 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 due to their flux lying below the current flux limits of X-ray instruments. This work focuses on identifying CT-AGNs hidden among mid-infrared (MIR) selected AGNs.

Methods. 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 the CT-AGN identification.

Results. Using MIR diagnostics, we found 7 to 23 CT-AGN candidates. Their subsequent X-ray stacking analysis revealed a clear detection at > 3σ significance in the soft band and only a > 1σ 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 ultimately constituted only 2.1% (23/1104) of our sample, significantly below the fraction predicted by CXB synthesis models. This indicates 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.