High redshift AGNs and HI reionisation: limits from the unresolved X-ray
  background
  The rapidly declining population of bright quasars at z~3 appears to make an
increasingly small contribution to the ionising background at the HI Lyman
limit. It is then generally though that massive stars in (pre-)galactic systems
may provide the additional ionising flux needed to complete HI reionisation by
z>6. A galaxy dominated background, however, may require that the escape
fraction of Lyman continuum radiation from high redshift galaxies is as high as
10%, a value somewhat at odds with (admittedly scarce) observational
constraints. High escape fractions from dwarf galaxies have been advocated, or,
alternatively, a so-far undetected (or barely detected) population of
unobscured, high-redshift faint AGNs. Here we question the latter hypothesis,
and show that such sources, to be consistent with the measured level of the
unresolved X-ray background at z=0, can provide a fraction of the HII filling
factor not larger than 13% by z=6. The fraction rises to <27% in the somewhat
extreme case of a constant comoving redshift evolution of the AGN emissivity.
This still calls for a mean escape fraction of ionising photons from high-z
galaxies >10%.
