All the recent observational data on quasars
and AGNs, in combination with theoretical studies of their
evolution and the accretion processes that produce both
their luminosity and growth in mass, are now enabling new
global studies of their history (e.g.,
Yu & Tremaine 2002;
Yu 2003;
Steed, Weinberg, & Miralda-Escudé, in preparation).
The goal is to determine how an initial black hole mass
function evolves into the one observed today in the local Universe
by considering the continuity equation and how the the masses
grow with accretion processes. The simple equation L =
(
/
Edd)M
c2, where L is
the luminosity produced by an accretion rate
in Eddington units
with efficiency for
a black hole of mass M, tells
us that if we could observationally determine L and
along with black hole masses, for example, we would have enough
information to model the evolution of the black holes in galaxies.
Put another way, the general goal is to combine the black hole
mass function, the time history of accretion, and the distribution of
accretion rates and efficiencies to see if we can match the
observed luminosity and mass functions for AGNs and black holes. One
immediate problem at present is that we do not have a way of
separately estimating
and
/
Edd; typically
people
assume that is 0.1
or some range of values depending
on the accretion models they adopt. Another problem is accounting
properly for the number of obscured sources in flux-limited samples.
Nonetheless, there are enough existing data to permit interesting progress on the problem. For example, the combination of the black hole mass function for local galaxies and the X-ray background provide integral constraints that must be satisfied by any model. The mass function represents the end point of the accretion processes, while the X-ray background provides a measure of the integrated luminosity produced by accretion over the history of the Universe. The improved optical data on the quasar luminosity functions provide additional constraints on how and when this all occurred, because they map out the evolution of the emitted light with cosmic time. At the same time, the deep X-ray and radio surveys and related optical observations provide crucial information on the contribution of obscured sources to the accretion history of the Universe.
Yu & Tremaine (2002)
find that the quasar luminosity functions
and local black hole mass functions are consistent if
0.1 and the black
hole mass
growth occurred during the optically bright phase. The lifetime
of luminous quasars would be of order 108 years. At the same
time, there remain important questions about the accretion efficiency
of lower luminosity quasars and AGNs and its dependence on accretion
rate, for example.