The Impact of Dust in Host Galaxies on Quasar Luminosity Functions
  We have investigated effects of dust attenuation on quasar luminosity
functions using a semi-analytic galaxy formation model combined with a large
cosmological N-body simulation. We estimate the dust attenuation of quasars
self-consistently with that of galaxies by considering the dust in their host
bulges. We find that the luminosity of the bright quasars is strongly dimmed by
the dust attenuation, about 2 mag in the B-band. Assuming the empirical
bolometric corrections for active galactic nuclei (AGNs) by Marconi et al., we
find that this dust attenuation is too strong to explain the B-band and X-ray
quasar luminosity functions simultaneously. We consider two possible mechanisms
that weaken the dust attenuation. As such a mechanism, we introduce a time
delay for AGN activity, that is, gas fueling to a central black hole starts
some time after the beginning of the starburst induced by a major merger. The
other is the anisotropy in the dust distribution. We find that in order to make
the dust attenuation of the quasars negligible, either the gas accretion into
the black holes has to be delayed at least three times the dynamical timescale
of their host bulges or the dust covering factor is as small as 0.1.
