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dark ages
reionization
first stars
diffuse radiation
intergalactic medium
quasars: absorption lines
Astrophysics -
Astrophysics of Galaxies
Astrophysics - Cosmology and Nongalactic
Astrophysics
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 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 (∆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 }}}He{{II}}} of a fluctuating UV background from the median {τ }eff}, finding a factor ≃5 decrease in {{{Γ }}}He{{II}}} between z ≃ 2.6 and z ≃ 3.1. At z ≃ 3.1, {{{Γ }}}He{{II}}}=≤ft[{9.1}-1.2+1.1 ({stat}.){ }-3.4+2.4 ({sys}.)\right]× {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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with programs 13013 and 13875. Archival Hubble Space Telescope data (programs 7575, 9350, 11528, 12033, 12178, 12249, 12816) were obtained from the Mikulski Archive for Space Telescopes (MAST).
