Towards a complete census of luminous Compton-thick Active Galactic Nuclei in the local Universe

TL;DR

This study systematically investigates the prevalence of Compton-thick AGN in the local universe using IR-selected samples and X-ray observations, revealing a higher fraction than previously detected and no evolution with luminosity.

Contribution

It provides the first comprehensive estimate of Compton-thick AGN fraction in the local universe using IR selection and new X-ray data, including previously unexplored observations.

Findings

Compton-thick AGN constitute 25-30% of local IR-selected AGN.

Uncovered four new Compton-thick sources with XMM-Newton and NuSTAR.

No evidence found for evolution of Compton-thick fraction with luminosity.

Abstract

X-ray surveys provide the most efficient means for the detection of Active Galactic Nuclei (AGN). However, they face difficulties in detecting the most heavily obscured Compton-thick AGN. The BAT detector on board the Gehrels/Swift mission, operating in the very hard 14-195 keV band, has provided the largest samples of Compton-thick AGN in the local Universe. However, even these flux limited samples can miss the most obscured sources among the Compton-thick AGN population. A robust way to find these local sources is to systematically study volume-limited AGN samples detected in the IR or the optical part of the spectrum. Here, we utilize a local sample (<100 Mpc) of mid-IR selected AGN, unbiased against obscuration, to determine the fraction of Compton-thick sources in the local universe. When available we acquire X-ray spectral information for the sources in our sample from previously published studies. Additionally, to maximize the X-ray spectral information for the sources in our sample, we analyse, for the first time, eleven unexplored XMM-Newton and NuSTAR observations, identifying four new Compton-thick sources. Our results reveal an increased fraction of Compton-thick AGN among the sources that have not been detected by BAT of 44 %. Overall we estimate a fraction of Compton thick sources in the local universe of 25-30% among mid-IR selected AGN. We find no evidence for evolution of the AGN Compton-thick fraction with luminosity.