Mass without radiation: heavily obscured AGN, the X-ray Background and the Black Hole Mass Density

TL;DR

This paper explores how increasing the estimated local black hole mass density impacts our understanding of AGN growth, suggesting that heavily obscured, Compton thick AGN could account for more black hole mass without conflicting with X-ray and infrared background constraints.

Contribution

It demonstrates that a larger fraction of heavily obscured AGN can explain the increased black hole mass density without violating background spectral energy density limits.

Findings

Heavily obscured AGN can significantly contribute to black hole growth.

The increased black hole mass density is compatible with background spectral constraints.

Radiatively inefficient accretion channels may not be necessary to explain the data.

Abstract

A recent revision of black hole scaling relations (Kormendy & Ho 2013), indicates that the local mass density in black holes should be increased by up to a factor of five with respect to previously determined values. The local black hole mass density is connected to the mean radiative efficiency of accretion through the time integral of the AGN volume density and a significant increase of the local black holes 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, given the intrinsic degeneracies in the Soltan argument, this solution is not unique. Here we show how it is possible to accommodate a larger fraction of heavily buried, Compton thick AGN, without violating the limit imposed by the hard X-ray and mid-infrared backgrounds spectral energy density.