The redshift evolution of escape fraction of hydrogen ionizing photons
  from galaxies
  Using our cosmological radiative transfer code, we study the implications of
the updated QSO emissivity and star formation history for the escape fraction
(f_esc) of hydrogen ionizing photons from galaxies. We estimate the f_esc that
is required to reionize the Universe and to maintain the ionization state of
the intergalactic medium in the post-reionization era. At z>5.5, we show that a
constant f_esc of 0.14 to 0.22 is sufficient to reionize the Universe. At
z<3.5, consistent with various observations, we find that f_esc can have values
from 0 to 0.05. However, a steep rise in f_esc, of at least a factor of ~3, is
required between z=3.5 to 5.5. It results from a rapidly decreasing QSO
emissivity at z>3 together with a nearly constant measured H I photoionization
rates at 3<z<5. We show that, this requirement of a steep rise in f_esc over a
very short time can be relaxed if we consider the contribution from a recently
found large number density of faint QSOs at z>4. In addition, a simple
extrapolation of the contribution of such QSOs to high-z suggests that QSOs
alone can reionize the Universe. This implies, at z>3.5, that either the
properties of galaxies should evolve rapidly to increase the f_esc or most of
the low-mass galaxies should host massive black holes and sustain accretion
over a prolonged period. These results motivate a careful investigation of
theoretical predictions of these alternate scenarios that can be distinguished
using future observations. Moreover, it is also very important to revisit the
measurements of H I photoionization rates that are crucial to the analysis
presented here.
