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

TL;DR

This study calibrates cosmological radiative transfer simulations with Lyman alpha data, revealing large-scale UV background fluctuations at high redshift likely caused by rare bright sources, impacting reionisation models.

Contribution

It introduces a calibration of radiative transfer simulations with observational data and highlights the role of rare bright sources in UV background fluctuations during reionisation.

Findings

Models match observed reionisation timing and Lyman alpha evolution.

Simulations fail to reproduce high-opacity tail at z > 5.

Large-scale UV fluctuations suggest influence of rare bright sources.

Abstract

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.