On the relative Contribution of high-redshift Galaxies and Active
  Galactic Nuclei to Reionization
  In this paper we discuss the contribution of different astrophysical sources
to the ionization of neutral hydrogen at different redshifts. We critically
revise the arguments in favour/against a substantial contribution of Active
Galactic Nuclei (AGNs) and/or Lyman Break Galaxies (LBGs) to the reionization
of the Universe at z>5. We consider extrapolations of the high-z QSO and LBG
luminosity functions and their redshift evolution as well as indirect
constraints on the space density of lower luminosity Active Galactic Nuclei
based on the galaxy stellar mass function. Since the hypothesis of a
reionization due to LBGs alone requires a significant contribution of faint
dwarf galaxies and a LyC photon escape fraction (f_esc) of the order of ~20%,
in tension with present observational constraints, we examine under which
hypothesis AGNs and LBGs may provide a combined relevant contribution to the
reionization. We show that a relatively steep faint-end of the AGN luminosity
function, consistent with present constraints, provides a relevant (although
sub-dominant) contribution, thus allowing us to recover the required ionizing
photon rates with f_esc~5% up to z~7. At higher redshifts, we test the case for
a luminosity-dependent f_esc scenario and we conclude that, if the observed
LBGs are indeed characterized by very low f_esc, values of the order of
f_esc~70% are needed for objects below our detection threshold, for this galaxy
population to provide a substantial contribution to reionization. Clearly, the
study of the properties of faint sources (both AGNs and LBGs) is crucial.
