Calibrating cosmological radiative transfer simulations with Ly α forest data: evidence for large spatial UV background fluctuations atz∼ 5.6–5.8 due to rare bright sources

We calibrate here cosmological radiative transfer simulations with aton/ramses with a range of measurements of the Ly α opacity from Quasi-Stellar Objects (QSO) absorption spectra. We find the Ly α opacity to be very sensitive to the exact timing of hydrogen reionization. Models reproducing the measured evolution of the mean photoionization rate and average mean free path reach overlap at z ∼ 7 and predict an accelerated evolution of the Ly α opacity at z > 6 consistent with the rapidly evolving luminosity function of Ly α 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 Ly α opacity PDF (probability distribution function) at z > 5. We argue that this is due to spatial UV fluctuations in the post-overlap phase of reionization on substantially larger scales than predicted by our source model, where the ionizing emissivity is dominated by large numbers of sub-L* galaxies. We further argue that this suggests a significant contribution to the ionizing UV background by much rarer bright sources at high redshift.