PRIMUS: An observationally motivated model to connect the evolution of the AGN and galaxy populations out to z~1. (arXiv:1301.1689v1 [astro-ph.CO]):
We present an observationally motivated model to connect the AGN and galaxy
populations at 0.2<z<1.0 and predict the AGN X-ray luminosity function (XLF).
We start with measurements of the stellar mass function of galaxies (from the
Prism Multi-object Survey) and populate galaxies with AGNs using models for the
probability of a galaxy hosting an AGN as a function of specific accretion rate
(the rate of supermassive black hole growth scaled relative to the host stellar
mass). Our model is based on measurements indicating that the specific
accretion rate distribution is a universal function across a wide range of host
stellar mass with slope gamma_1=0.65 and an overall normalization that evolves
strongly with redshift. We test several simple assumptions to extend this model
to high specific accretion rates (beyond the measurements) and compare the
predictions for the XLF with the observed data. We find good agreement with a
model that allows for a break in the specific accretion rate distribution at a
point corresponding to the Eddington limit, with a steep power-law tail to
super-Eddington ratios with slope gamma_2=-2.1 +0.3 -0.5. We convert between
specific accretion rate and Eddington ratio by assuming a scaling between black
hole mass and host stellar mass with an intrinsic scatter of +/-0.38 dex. Our
results show that samples of low luminosity AGNs are dominated by moderately
massive galaxies (M* ~ 10^{10-11} M_sun) growing with a wide range of accretion
rates -- a consequence of the shape of the galaxy stellar mass function rather
than a preference for AGN activity at a particular stellar mass. The observed
population of the most luminous AGN may be severely skewed to the most extreme
sources with elevated black hole masses relative to their host galaxies and in
rare phases of very rapid accretion.
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