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.
Date added: Wed, 16 Oct 13