2019ApJ...875..111W

We present measurements of the large-scale (≃40 comoving Mpc) effective optical depth of He II Lyα absorption, τ_eff, 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 τ_eff 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 τ_eff in 24 He II sightlines. We confirm an increase of the median τ_eff from ≃2 at z = 2.7 to τ_eff≳5 at z > 3, and a scatter in τ_eff 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 τ_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 He II photoionization rate Γ_HeII of a fluctuating UV background from the median τ_eff, finding a factor ≃5 decrease in Γ_HeII between z ≃ 2.6 and z ≃ 3.1. At z ≃ 3.1, Γ_HeII=[9.1_–1.2^+1.1(stat.)_–3.4^+2.4(sys.)]×10^–16 s^–1 corresponds to a median He II fraction of ≃2.5%, indicating that our data probe the tail end of He II reionization.