The effect† of helium reionization on the Lyα forest hydrogen flux statistics

We assess the impact of QSOs on the high redshift (z > 4) Intergalactic Medium using Monte Carlo realisations of QSO populations and the He 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 i Lyα optical depth distributions at z > 5, much broader distributions are found for the higher numbers of QSOs based on x-ray selection, suggesting a large QSO contribution to the ultra-violet background at z > 5 may offer an alternative to late reionization models to account for the broad H i Lyα optical depth distributions. Realisations using QSOs based on the higher QSO counts also much better recover the measured pixel flux auto-correlation function at z > 5. The He iii regions from QSO sources according to both types of luminosity function suppress the pixel flux power spectrum on small scales, k > 0.02 s km⁻¹, while enhancing it on larger, both by amounts of up to tens of percent at z > 4, with the magnitude increasing with the intergalactic He iii filling factor and the boost in temperature within the He iii regions.