Speaker
Description
A key challenge to the standard cosmological model is the observation
of very massive (M > 10^9 Msun) and luminous (L > 10^{13} Lsun)
quasars already in place by z~7, when the age of the universe was
just ~800 Myr. The formation of such supermassive black holes (SMBH)
cannot generally occur in the time available if starting from
stellar mass black holes. We explore implications of the
idea of direct collapse black holes (DCBH) emerging from the collapse
of a special class of supermassive stars (~ 10^5 Msun) that could only
form at high redshift in so-called atomic cooling halos. Both numerical
and semi-analytic modeling implies a brief period of rapidly growing
production of supermassive stars, in the approximate redshift range of
z~20 to z~13. Our work shows that the mass function of SMBHs after a
limited time period of rapid formation of DCBH, coupled with a
super-Eddington accretion from their host halos, can be described
as a tapered power law function. The power law at intermediate
masses has an index that is the dimensionless ratio alpha = lambda/gamma,
where lambda is the growth rate of the number density of DCBHs during
their formation era, and gamma is the growth rate of DCBH masses by
super-Eddington accretion during the same growth era. A second feature
is a break in the power-law profile at high masses, above which
the mass function declines rapidly. This mass distribution is largely
set during this early growth era, and subsequent mass growth may be more
limited. We also calculate the implied luminosity function of the
resulting SMBH, which is in remarkable agreement with the broken power
law quasar luminosity function that has been observed at high redshift.
