Abstract
We present a simple model for the relationship between quasars, galaxies, and
dark matter halos from 0<z<5. In the model, black hole (BH) mass is linearly
related to galaxy mass, and galaxies are connected to dark matter halos via
empirically constrained relations. A simple "scattered" light bulb model for
quasars is adopted, wherein BHs shine at a fixed fraction of the Eddington
luminosity during accretion episodes, and Eddington ratios are drawn from a
lognormal distribution that is redshift-independent. This model has two free,
physically meaningful parameters at each redshift, the normalization of the
M_BH-M_gal relation and the quasar duty cycle; these parameters are fit to the
observed quasar luminosity function (LF) over the interval 0.5<z<4.75. This
simple model provides an excellent fit to the LF at all epochs, and also
successfully predicts the observed projected two-point correlation of quasars
from 0.5<z<2.5. It is significant that a single quasar duty cycle at each
redshift is capable of reproducing the extant observations. The data are
therefore consistent with a scenario wherein quasars are equally likely to
exist in galaxies, and therefore dark matter halos, over a wide range in
masses. The knee in the quasar LF is a reflection of the knee in the stellar
mass-halo mass relation. The normalization of the M_BH-M_gal relation increases
as (1+z)^2, consistent with observations, although in our model the
normalization is degenerate with the mean Eddington ratio. Future constraints
on the quasar LF and quasar clustering at high redshift will provide strong
constraints on the model.
Description
[1208.3198] A Simple Model for Quasar Demographics
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