your ID: 0191 
First Name: Xinwu 
Last Name: Cao 
Affiliation: Shanghai Astronomical Observatory, Chinese Academy of Sciences 
Title: Growth of massive black holes during radiatively inefficient accretion phases 
Authors: Xinwu Cao 
Topic: 1 
Session: oral 
Abstract: There is evidence that the growth of massive black holes is mainly through accreting 
surrounded gases.  The massive black holes in most faint active galactic nuclei (AGNs) and 
even some normal galaxies are still accreting gases, though their accretion rates 
could be very low. When the accretion rate $\\dot{m}$ is lower than the critical value 
$\\dot{m}_{\\rm crit}$ ($\\dot{m}=\\dot{M}/\\dot{M}_{\\rm Edd}$), the transition of 
a standard thin disk to a radiatively inefficient accretion flow (RIAF) is expected to occur, 
The RIAF is very hot, and it radiates mostly in the hard X-ray band. We derive the black hole mass 
density as a function of redshift from the hard X-ray luminosity function of AGNs assuming massive 
black holes grow via accreting surrounded gases. Both the contributions of bright AGNs and 
the RIAFs in faint AGNs/normal galaxies to the X-ray background (XRB) are calculated. Comparing 
with the observed XRB, we find that about 5 per cent of local black hole mass density came 
from radiatively inefficient accretion phases, if the black holes are spinning rapidly at 
$a\\sim 0.9$.