A large fraction of rapidly-growing supermassive black holes evade census

Abstract

Dedicated searches suggest that the fraction of obscured Active Galactic Nuclei (AGN) decreases substantially with increasing luminosity. To explain these findings receding torus models have often been adopted. I shall present the results of a recent study where we determined the intrinsic fraction of optical type-2 AGN at z < 1 and X-ray luminosities from 1042 to 1045 erg s−1. We used a complete X-ray selected sample of 199 AGN, from the Bright Ultrahard XMM-Newton Survey, and the distributions of covering factors of AGN tori derived from CLUMPY torus models. Since these distributions combined over the total AGN population need to match the intrinsic type-2 AGN fraction, we revealed a population of X-ray undetected objects with high-covering factor tori, which are increasingly numerous at higher AGN luminosities. When these “missing” objects are included, we found that Compton-thick AGN account at most for ∼ 35 W of the total population. The intrinsic type-2 AGN fraction is ∼ 58 W and has a weak, non-significant luminosity dependence. Our findings imply that the majority of luminous rapidly-accreting supermassive black holes at z < 1 reside in highly-obscured nuclear environments but most of them are so deeply embedded that they have so far escaped detection in X-rays in < 10 keV wide-area surveys.