The space density of ultra-luminous QSOs at the end of reionization
  epoch by the QUBRICS Survey and the AGN contribution to the hydrogen ionizing
  background
  Motivated by evidences favoring a rapid and late hydrogen reionization
process completing at z~5.2-5.5 and mainly driven by rare and luminous sources,
we have reassessed the estimate of the space density of ultra-luminous QSOs at
z~5 in the framework of the QUBRICS survey. A ~90% complete sample of 14
spectroscopically confirmed QSOs at M1450<-28.3 and 4.5<z<5.0 has been derived
in an area of 12,400 sq. deg., thanks to multi-wavelength selection and GAIA
astrometry. The space density of z~5 QSOs within -29.3<M1450<-28.3 is three
times higher than previous determinations. Our results suggest a steep
bright-end slope for the QSO luminosity function at z~5 and a mild redshift
evolution of the space density of ultra-bright QSOs (M1450~-28.5) at 3<z<5.5,
in agreement with the redshift evolution of the much fainter AGN population at
M1450~-23. These findings are consistent with a pure density evolution for the
AGN population at z>3. Adopting our z~4 QSO luminosity function and applying a
mild density evolution in redshift, a photo-ionization rate of
GammaHI=0.46+0.17-0.09x10^-12 s^-1 has been obtained at z=4.75, assuming an
escape fraction of ~70% and a steep faint-end slope of the AGN luminosity
function. The derived photo-ionization rate is ~50-100% of the ionizing
background measured at the end of the reionization epoch, suggesting that AGNs
could play an important role in the cosmological reionization process.
