The Evolution and Eddington Ratio Distribution of Compton Thick Active
  Galactic Nuclei
  Previous studies of the active galactic nuclei (AGN) contribution to the
cosmic X-ray background (CXB) consider only observable parameters such as
luminosity and absorbing column. Here, for the first time, we extend the study
of the CXB to physical parameters including the Eddington ratio of the sources
and the black hole mass. In order to calculate the contribution to the CXB of
AGN accreting at various Eddington ratios, an evolving Eddington ratio space
density model is calculated. In particular, Compton thick (CT) AGN are modeled
as accreting at specific, physically motivated Eddington ratios instead of as a
simple extension of the Compton thin type 2 AGN population. Comparing against
the observed CT AGN space densities and log N-log S relation indicates that CT
AGN are likely a composite population of AGN made up of sources accreting
either at >90% or <1% of their Eddington rate.
