Constraining the UV emissivity of AGN throughout cosmic time via X-ray
  surveys
  The cosmological process of hydrogen (HI) reionization in the intergalactic
medium is thought to be driven by UV photons emitted by star-forming galaxies
and ionizing active galactic nuclei (AGN). The contribution of QSOs to HI
reionization at $z>4$ has been traditionally believed to be quite modest.
However, this view has been recently challenged by new estimates of a higher
faint-end UV luminosity function (LF). To set firmer constraints on the
emissivity of AGN at $z<6$, we here make use of complete X-ray selected samples
including deep Chandra and new COSMOS data, capable to efficiently measure the
1 ryd comoving AGN emissivity up to $z\sim5-6$ and down to five magnitudes
fainter than probed by current optical surveys, without any luminosity
extrapolation. We find good agreement between the logN$\rm{_H}\lesssim21-22$
cm$^{-2}$ X-ray LF and the optically-selected QSO LF at all redshifts for
$M_{1450}\leq -23$. The full range of the logN$\rm{_H}\lesssim21-22$ cm$^{-2}$
LF ($M_{1450} \leq -17$) was then used to quantify the contribution of AGN to
the photon budget critical value needed to keep the Universe ionized. We find
that the contribution of ionizing AGN at $z = 6$ is as small as 1\% - 7\%, and
very unlikely to be greater than 30\%, thus excluding an AGN-dominated
reionization scenario.
