Mass without radiation: Heavily obscured AGNs, the X-ray background, and the black hole mass density

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: andrea.comastri@oabo.inaf.it
² Dipartimento di Fisica e Astronomia, Università di Firenze, via Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 10 December 2014
Accepted: 13 January 2015

Abstract

A recent revision of black hole scaling relations indicate that the local mass density in black holes should be five times higher than previously determined values. The local black hole mass density is connected to the mean radiative efficiency of accretion through the time integral of the Active Galactic Nuclei (AGN) volume density and a significant increase in the local black hole mass density would have interesting consequences on AGN accretion properties and demography. One possibility to explain a large black hole mass density is that most of the black hole growth is via radiatively inefficient channels such as super Eddington accretion; however, this solution is not unique. Here we show how it is possible to accommodate a larger fraction of heavily buried, Compton-thick AGNs, without violating the limit imposed by the spectral energy density of the hard X-ray and mid–infrared backgrounds.