The effect of helium reionization on the Ly α forest hydrogen flux statistics

ABSTRACT We assess the impact of quasi-stellar objects (QSOs) on the high-redshift ($z\gt 4$) intergalactic medium using Monte Carlo realizations of QSO populations and the He $\rm \scriptstyle III$ regions they generate, applied to the Sherwood–Relics simulations, allowing for uncertainties in the QSO luminosity function, its evolution, and QSO spectra and ages. While QSO luminosity functions based on optical–infrared selection are unable to reproduce the broadening H $\rm \scriptstyle I$ Ly $\alpha$ optical depth distributions at $z\gt 5$, much broader distributions are found for the higher numbers of QSOs based on X-ray selection, suggesting a large QSO contribution to the ultraviolet background at $z\gt 5$ may offer an alternative to late reionization models to account for the broad H $\rm \scriptstyle I$ Ly $\alpha$ optical depth distributions. Realizations using QSOs based on the higher QSO counts also much better recover the measured pixel flux autocorrelation function at $z\gt 5$. The He $\rm \scriptstyle III$ regions from QSO sources according to both types of luminosity function suppress the pixel flux power spectrum on small scales, $k\gt 0.02\, \mathrm{s\, km^{-1}}$, while enhancing it on larger, both by amounts of up to tens of per cent at $z\gt 4$, with the magnitude increasing with the intergalactic He $\rm \scriptstyle III$ filling factor and the boost in temperature within the He $\rm \scriptstyle III$ regions.