Towards a complete census of luminous Compton-thick active galactic nuclei in the Local Universe

¹ Institute for Astronomy Astrophysics Space Applications and Remote Sensing (IAASARS), National Observatory of Athens, Ioannou Metaxa & Vasileos Pavlou, Penteli 15236, Greece
² School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
³ Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA
⁴ Physics Department, University of Durham, South Road, Durham DH1 3LE, UK

^⋆ Corresponding author; ig@noa.gr

Received: 16 August 2024
Accepted: 7 October 2024

Abstract

X-ray surveys provide the most efficient means for the detection of active galactic nuclei (AGNs). However, they do face difficulties in detecting the most heavily obscured Compton-thick AGNs. 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 may 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 utilise a local sample (< 100 Mpc) of mid-IR-selected AGNs, unbiased against obscuration, to determine the fraction of Compton-thick sources in the Local Universe. When available, we acquired X-ray spectral information for the sources in our sample from previously published studies. In addition, to maximise the X-ray spectral information for the sources in our sample, we analysed eleven unexplored XMM-Newton and NuSTAR observations, for the first time. In this way, we identified four new Compton-thick sources. Our results reveal an increased fraction of Compton-thick AGNs among the sources that have not been detected by BAT of 44%. Overall, we have estimated a 25–30% share of Compton-thick sources in the Local Universe among mid-IR-selected AGNs. We find no evidence for any evolution of the AGN Compton-thick fraction with luminosity.