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