The Evolution of the He ii-ionizing Background at Redshifts 2.3 < z < 3.8 Inferred from a Statistical Sample of 24 HST/COS He ii Lyα Absorption Spectra*

Abstract

Abstract We present measurements of the large-scale (≈40 comoving Mpc) effective optical depth of He ii Lyα absorption, , at 2.54 < z < 3.86 toward 16 He ii-transparent quasars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope, to characterize the ionization state of helium in the intergalactic medium (IGM). We provide the first statistical sample of measurements in six signal-to-noise ratio ≳3 He ii sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of in 24 He ii sightlines. We confirm an increase of the median from ≃2 at z = 2.7 to at z > 3, and a scatter in that increases with redshift. The z > 3.5 He ii absorption is predominantly saturated, but isolated narrow (Δv < 650 km s−1) 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 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 He ii photoionization rate of a fluctuating UV background from the median , finding a factor ≃5 decrease in between z ≃ 2.6 and z ≃ 3.1. At z ≃ 3.1, s−1 corresponds to a median He ii fraction of ≃2.5%, indicating that our data probe the tail end of He ii reionization.