18 février 2020Evidence for a significant contribution of rare bright sources to the ionising UV background at z~5.6-5.8

I will present cosmological radiative transfer simulations of Hydrogen reionization calibrated with a range of measurements of the Lyα opacity from QSO absorption spectra. I will show that the Lyα opacity is 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 Lyα opacity at z > 6 consistent with the rapidly evolving luminosity function of Lyα 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 Lyα opacity PDF at z > 5. I will thus present a toy model arguing that this could be 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. This, furthermore emphasize that there should be a significant contribution to the ionising UV background by much rarer bright sources at high redshift.