Faint-end Quasar Luminosity Functions from Cosmological Hydrodynamic
  Simulations
  We investigate the predictions for the faint-end quasar luminosity function
(QLF) and its evolution using fully cosmological hydrodynamic simulations which
self-consistently follow star formation, black hole growth and associated
feedback processes. We find remarkably good agreement between predicted and
observed faint end of the optical and X-ray QLFs (the bright end is not
accessible in our simulated volumes) at z < 2. At higher redshifts our
simulations tend to overestimate the QLF at the faintest luminosities. We show
that although the low (high) luminosity ranges of the faint-end QLF are
dominated by low (high) mass black holes, a wide range of black hole masses
still contributes to any given luminosity range. This is consistent with the
complex lightcurves of black holes resulting from the detailed hydrodynamics
followed in the simulations. Consistent with the results on the QLFs, we find
good agreement for the evolution of the comoving number density (in optical,
soft and hard X-ray bands) of AGN for luminosities above 10^43 erg/s. However,
the luminosity density evolution from the simulation appears to imply a peak at
higher redshift than constrained from hard X-ray data (but not in optical). Our
predicted excess at the faintest fluxes at z >= 2 does not lead to an
overestimate to the total X-ray background and its contribution is at most a
factor of two larger than the unresolved fraction of the 2-8 keV background.
Even though this could be explained by some yet undetected, perhaps heavily
obscured faint quasar population, we show that our predictions for the faint
sources at high redshifts (which are dominated by the low mass black holes) in
the simulations are likely affected by resolution effects.
