Identifying Compton-thick AGNs in the COSMOS

I. Among X-ray AGNs with low photon counts

¹ School of Mathematics and Physics, 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; guoxiaotong@aqnu.edu.cn; qsgu@nju.edu.cn

Received: 30 August 2024
Accepted: 15 January 2025

Abstract

Context. Compton-thick active galactic nuclei (CT-AGNs), characterized by a significant absorption with column densities of N_H ≥ 1.5 × 10²⁴ cm⁻², emit feeble X-ray radiation and are even undetectable by X-ray instruments, making them difficult to identify. X-ray radiation from AGNs is the predominant source of the cosmic X-ray background (CXB). Based on AGN synthesis models for the CXB, the fraction of CT-AGNs should constitute a substantial portion of AGN population, approximately 30% or more.

Aims. The fraction of CT-AGNs discovered in the Cosmological Evolution Survey (COSMOS) is significantly lower than this value. This means that many CT-AGNs may be hidden in AGNs that exhibit low photon counts or that have not been detected by X-ray instruments. This work focuses on identifying CT-AGNs hidden in AGNs with low photon counts.

Methods. Firstly, we selected 440 AGNs with abundant multiwavelength data as our sample. Secondly, we analyzed multiwavelength data, extracting crucial physical parameters required for the CT-AGN diagnosis. Finally, we used multiwavelength approaches to identify CT-AGNs.

Results. We have successfully identified 18 CT-AGNs in our sample. Among the CT-AGNs, four AGNs show discrepant results across different diagnostic methods. We discuss the potential reasons behind these diagnostic discrepancies. We explore the impact of estimating [O III]λ 5007 luminosities based on [O II]λ 3727 luminosities for the CT-AGN diagnosis. We have also found that the properties of host galaxies for CT-AGNs and non-CT-AGNs do not show significant discrepancies.