Calibrating cosmological radiative transfer simulations with Lyman alpha
  forest data: Evidence for large spatial UV background fluctuations at z ~ 5.6
  - 5.8 due to rare bright sources
  We calibrate here cosmological radiative transfer simulation with ATON/RAMSES
with a range of measurements of the Lyman alpha opacity from QSO absorption
spectra. We find the Lyman alpha opacity to be very sensitive to the exact
timing of hydrogen reionisation. Models reproducing the measured evolution of
the mean photoionisation rate and average mean free path reach overlap at z ~ 7
and predict an accelerated evolution of the Lyman alpha opacity at z > 6
consistent with the rapidly evolving luminosity function of Lyman alpha
emitters in this redshift range. Similar to "optically thin" simulations our
full radiative transfer simulations fail, however, to reproduce the
high-opacity tail of the Lyman alpha opacity PDF at z > 5. We argue that this
is due to spatial UV fluctuations in the post-overlap phase of reionisation on
substantially larger scales than predicted by our source model, where the
ionising emissivity is dominated by large numbers of sub-L* galaxies. We
further argue that this suggests a significant contribution to the ionising UV
background by much rarer bright sources at high redshift.
