The Evolution of the HeII-Ionizing Background at Redshifts 2.3<z<3.8 Inferred from a Statistical Sample of 24 HST/COS HeII Ly$α$ Absorption Spectra

Abstract

We present measurements of the large-scale (~40 comoving Mpc) effective optical depth of HeII Ly$α$ absorption, $τ_{\rm eff}$, at 2.54 3 HeII sightlines at z>3.5, and study the redshift evolution and sightline-to-sightline variance of $τ_{\rm eff}$ in 24 HeII sightlines. We confirm an increase of the median $τ_{\rm eff}$ from ~2 at z=2.7 to >5 at z>3, and a scatter in $τ_{\rm eff}$ that increases with redshift. The z>3.5 HeII absorption is predominantly saturated, but isolated narrow ($Δv<650$ km/s) transmission spikes indicate patches of reionized helium. We compare our measurements to predictions for a range of UV background models applied to outputs of a large-volume (146 comoving Mpc)$^3$ hydrodynamical simulation by forward-modeling our sample's quality and size. At z>2.74 the variance in $τ_{\rm eff}$ significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median HeII photoionization rate $Γ_{\rm HeII}$ of a fluctuating UV background from the median $τ_{\rm eff}$, finding a factor ~5 decrease in $Γ_{\rm HeII}$ between z~2.6 and z~3.1. At z~3.1 a $Γ_{\rm HeII}=\left[9.1^{+1.1}_{-1.2}\,\mathrm{(stat.)}\,^{+2.4}_{-3.4}\,\mathrm{(sys.)}\right]\times10^{-16}\,\mathrm{s}^{-1}$ corresponds to a median HeII fraction of ~2.5%, indicating that our data probe the tail end of HeII reionization.