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

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 (COS) on the Hubble Space Telescope (HST), 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 (∆v < 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 a Γ HeII = 9.1 +1.1 −1.2 (stat.) +2.4 −3.4 (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.

A consistent and robust measurement of the thermal state of the IGM at 2 ≤ z ≤ 4 from a large sample of Ly α forest spectra: evidence for late and rapid He ii reionization

“…The difference in the measured thermal parameters was hardly noticeable (< 0.8 percent). The range of possible He ii reionization histories consistent with the He ii opacity data is also rather small (Worseck et al 2019). Hence the effect of the differences in Jeans smoothing for the same instantaneous temperature and different He ii reionization histories is again expected to have a small effect on our measurements of thermal parameters.…”

“…The shape of the T0 − γ evolution is closely related to the He ii reionization history ( fHeII evolution Gaikwad et al 2019). It is noteworthy here that the reionization history used in the OH17; FG20 UVB models is calibrated to match the observed τ eff,HeII evolution from Worseck et al (2019). The good match between the shape of the observed T0 − γ evo-lution and that predicted by the UVB models with scaled photo-heating rates at z > 3 suggests that the T0 − γ evolution from our work is in good agreement with the observed τ eff,HeII evolution from Worseck et al (2019).…”

“…To calculate photo-ionization and photo-heating rates during He ii reionization, OH17; FG20 assume the He ii reionization history ( fHeII evolution) to be a free parametric function. The reionization history is calibrated to match the observed τ eff,HeII evolution (Worseck et al 2019). To compute the photo-heating rates during reionization, OH17; FG20 then use another free parameter namely the total heat injected during He ii reionization (∆THeII).…”

“…It is noteworthy here that the reionization history used in the OH17; FG20 UVB models is calibrated to match the observed τ eff,HeII evolution from Worseck et al (2019). The good match between the shape of the observed T0 − γ evo-lution and that predicted by the UVB models with scaled photo-heating rates at z > 3 suggests that the T0 − γ evolution from our work is in good agreement with the observed τ eff,HeII evolution from Worseck et al (2019). Such consistency is important because the evolution of T0 and γ at 2 ≤ z ≤ 4 is mainly driven by the fHeII evolution (see Fig.…”

“…Accurate measurements of thermal parameters and photo-ionization rates of the IGM have been used to place constraints on (i) the epoch and extent of H i and He ii reionization (Miralda-Escudé & Rees 1994;Theuns et al 2002b;Worseck et al 2011;Puchwein et al 2015;Worseck et al 2016;Upton Sanderbeck et al 2016;Gaikwad et al 2019;Worseck et al 2019; Upton Sanderbeck & Bird 2020), (ii) ionizing ultra-violet background (UVB) models that are important inputs for cosmological hydrodynamical simulations (McDonald & Miralda-Escudé 2001;Bolton et al 2005;Faucher-Giguère et al 2009;Haardt & Madau 2012;Khaire & Srianand 2015;Oñorbe et al 2017;Puchwein et al 2019; Faucher-Giguère 2020, hereafter HM12; KS15; OH17; KS19; P19; FG20 respectively), (iii) the escape fraction of H i ionizing photons from galaxies and the relative contribution of galaxies and QSOs to the total H i ionizing background (Faucher-Giguère et al 2008;Khaire et al 2016;Khaire 2017), (iv) and the effect of nonradiative processes like heating by cosmic rays (Nath & Biermann 1993;Samui et al 2005), blazars (Chang et al 2012;Puchwein et al 2012) and dark matter annihilation (Cirelli et al 2009;Liu et al 2020).…”