AGNs as main contributors to the UV ionizing emissivity at high
  redshifts: predictions from a Lambda-CDM model with linked AGN/galaxy
  evolution
  We have evaluated the contribution of the AGN population to the ionization
history of the Universe based on a semi-analytic model of galaxy formation and
evolution in the CDM cosmological scenario. The model connects the growth of
black holes and of the ensuing AGN activity to galaxy interactions. In the
model we have included a self consistent physical description of the escape of
ionizing UV photons; this is based on the blast-wave model for the AGN feedback
we developed in a previous paper to explain the distribution of hydrogen column
densities in AGNs of various redshifts and luminosities, due to absorption by
the host galaxy gas. The model predicts UV luminosity functions for AGNs which
are in good agreement with those derived from the observations especially at
low and intermediate redshifts (z=3). At higher redshifts (z>5) the model tends
to overestimate the data at faint luminosities. Critical biases both in the
data and in the model are discussed to explain such apparent discrepancies. The
predicted hydrogen photoionization rate as a function of redshift is found to
be consistent with that derived from the observations. All that suggests to
reconsider the role of the AGNs as the main driver of the ionization history of
the Universe.
