Faint AGNs at z>4 in the CANDELS GOODS-S field: looking for contributors
  to the reionization of the Universe
  In order to derive the AGN contribution to the cosmological ionizing
emissivity we have selected faint AGN candidates at $z>4$ in the CANDELS
GOODS-South field which is one of the deepest fields with extensive
multiwavelength coverage from Chandra, HST, Spitzer and various groundbased
telescopes. We have adopted a relatively novel criterion. As a first step high
redshift galaxies are selected in the NIR $H$ band down to very faint levels
($H\leq27$) using reliable photometric redshifts. This corresponds at $z>4$ to
a selection criterion based on the galaxy rest-frame UV flux. AGN candidates
are then picked up from this parent sample if they show X-ray fluxes above a
threshold of $F_X\sim 1.5\times 10^{-17}$ cgs (0.5-2 keV). We have found 22 AGN
candidates at $z>4$ and we have derived the first estimate of the UV luminosity
function in the redshift interval $4<z<6.5$ and absolute magnitude interval
$-22.5\lesssim M_{1450} \lesssim -18.5$ typical of local Seyfert galaxies. The
faint end of the derived luminosity function is about two/four magnitudes
fainter at $z\sim 4-6$ than that derived from previous UV surveys. We have then
estimated ionizing emissivities and hydrogen photoionization rates in the same
redshift interval under reasonable assumptions and after discussion of possible
caveats, the most important being the large uncertainties involved in the
estimate of photometric redshift for sources with featureless, almost power-law
SEDs and/or low average escape fraction of ionizing photons from the AGN host
galaxies. We argue that, under reasonable evaluations of possible biases, the
probed AGN population can produce at $z=4-6.5$ photoionization rates consistent
with that required to keep highly ionized the intergalactic medium observed in
the Lyman-$\alpha$ forest of high redshift QSO spectra, providing an important
contribution to the cosmic reionization.
