The Compton-thick active galactic nuclei luminosity function in the local Universe

A robust estimate combining Burst Alert Telescope detections and NuSTAR spectra

Institute for Astronomy Astrophysics Space Applications and Remote Sensing (IAASARS), National Observatory of Athens, Ioannou Metaxa & Vasileos Pavlou, Penteli 15236, Greece

^⋆ Corresponding author; ig@noa.gr

Received: 27 June 2024
Accepted: 10 December 2024

Abstract

Compton-thick active galactic nuclei (AGN) arguably constitute the most elusive class of sources, as they are absorbed by large column densities above 10²⁴ cm⁻². These extreme absorptions hamper the detection of the central source even in hard X-ray energies. In this work, we use both Neil Gehrels Swift and NuSTAR observations to derive the most accurate yet Compton-thick AGN luminosity function. We first compiled a sample of candidate Compton-thick AGN (N_H = 10^{24 − 25} cm⁻²) detected in the Swift Burst Alert Telescope (BAT) all-sky survey in the 14–195 keV band. We confirmed that they are Compton-thick sources by using the NuSTAR follow-up observations already presented in the literature. Our sample is composed of 44 sources, consistent with a column density of 10²⁴ − 10²⁵ cm⁻² at the 90% confidence level. These sources have intrinsic luminosities higher than L_{10 − 50 keV} ∼ 3 × 10⁴¹ erg s⁻¹ and are found up to a redshift of z = 0.05 (≈200 Mpc). We derived the luminosity function of Compton-thick AGN using a Bayesian methodology where both the full column density and the luminosity distributions are taken into account. The faint end of the luminosity function is flat, having a slope of γ₁ = 0.01_−0.74^+0.51, rather arguing against a numerous population of low luminosity Compton-thick AGN. Based on our luminosity function, we estimate that the fraction of Compton-thick AGN relative to the total number of AGN is of the order of 24 ± 5%, which is in agreement with previous estimates in the local Universe based on BAT samples.