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

Compton-thick active galactic nuclei (AGN) arguably constitute the most elusive class of sources, as they are absorbed by large column densities above 10 24 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 10 24 − 10 25 cm −2 at the 90% confidence level. These sources have intrinsic luminosities higher than L 10 − 50 keV ∼ 3 × 10 41 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 γ 1 = 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.