  We study the observed cosmic ionizing background as a constraint on the
nature of the sources responsible for the reionization of the Universe. In
earlier work, we showed that extrapolations of the Ultra-Violet Luminosity
Function (LF) of Lyman Break Galaxies (LBGs) at fixed Lyman continuum photon
escape fraction are not able to reproduce the redshift evolution of this
background. Here, we employ extrapolations of the high-z LFs to describe the
contribution of LBGs to the ionizing photon rate, taking into account the
smoothing of the baryonic perturbations, due to the background itself (i.e. the
filtering mass), as well as a possible sharp increase of the escape fraction in
dwarf galaxies. Under the hypothesis of a dominant contribution of LBGs to
cosmic reionization, our results suggest that sources fainter than the current
observational limits should be characterised by escape fractions of the order
of ~0.1-0.3 (larger than the current estimates for bright galaxies) to account
for a z>6 reionization and the measured evolution of cosmic ionizing
background, at the same time. The contribution to the background from quasars
turns out to be relevant at z<3. Overall, our results support the case for
dedicated observations of faint galaxies in the rest-frame UV, in order to
better determine their physical properties. Observed escape fractions outside
our proposed range bear relevant consequences on the nature of the
astrophysical sources responsible for cosmic reionization and/or its buildup
process.
