Sunday, 22 November 2009


Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030, China; Email:
accepted by ApJ

We derive the black hole mass density as a function of redshift from the hard X-ray AGN luminosity function
assuming that massive black holes grow via accreting the circumnuclear gases. The derived black hole mass
density matches the measured local black hole mass density at z = 0, which requires the average radiative
efficiency of AGNs to be ∼ 0.1−0.17. The massive black holes in most faint active galactic nuclei (AGNs) and
even normal galaxies are still accreting gases, though their accretion rates are very low. Radiatively inefficient
accretion flows (RIAFs) are supposed in these faint sources, which should radiate mostly in the hard X-ray
band. We calculate the contribution to the X-ray background from both the bright AGNs and the RIAFs in
faint AGNs/normal galaxies. Our calculations show that both the observed intensity and spectral shape of the
XRB with an energy peak at ∼ 30 keV can be well reproduced without including the emission of Comptonthick AGNs, if the massive black holes in faint AGNs/normal galaxies are spinning rapidly with a ∼ 0.9 and
accreting at rates m˙ ∼ 1.0 − 3.0×10
. It indicates that less than ∼5 per cent of local massive black hole mass
density was accreted during radiatively inefficient accretion phases, which is obviously only an upper limit,
because Compton-thick AGNs have not been considered. If the same number of the Compton-thick AGNs
with logNH = 24 − 25 as those with logNH = 23 − 24 is considered, the fraction of local black hole mass density
accumulated during inefficient accretion phases should be lower than ∼2 per cent. The constraints of the XRB
can provide upper limits on the average accretion rate for inactive galaxies.
Subject headings: galaxies: active—quasars: general—accretion, accretion disks—black hole physics; X-rays:
diffuse background