Local Black Hole Scaling Relations Imply Compton Thick or Super
  Eddington Accretion
  A recent analysis of black hole scaling relations, used to estimate the local
mass density in black holes, has indicated that the normalization of the
scaling relations should be increased by approximately a factor of five. The
local black hole mass density is connected to the mean radiative efficiency of
accretion through the time integral of the quasar volume density. The
correspondence between this estimate of the radiative efficiency and that
expected theoretically from thin-disk accretion has long been used as an
argument that most of the growth in black holes occurs via luminous accretion.
The increase of the mass density in black holes pushes the mean observed
radiative efficiency to values below that expected for thin-disk accretion for
any value of the black hole spin, including retrograde accretion disks. This
can be accommodated via black hole growth channels that are intrinsically
radiatively inefficient, such as super-Eddington accretion, or via growth
channels that are intrinsically radiatively efficient but for which few of the
photons are observed, such as Compton thick accretion. Measurements of the 30
keV peak in the X-ray background indicate a significant population of Compton
thick sources which can explain some, but not all, of the change in the local
black hole mass density. If this result is taken as evidence that
super-Eddington accretion is common, it greatly reduces the tension associated
with the growth timescale of observed z=7 quasars compared to the age of the
universe at that redshift.
