Astrophys. J., 875, 111-111 (2019/April-3)
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.
WORSECK G., DAVIES F.B., HENNAWI J.F. and PROCHASKA J.X.
Abstract (from CDS):
We present measurements of the large-scale (≃40 comoving Mpc) effective optical depth of He II Lyα absorption, τeff, 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 τeff 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 τeff in 24 He II sightlines. We confirm an increase of the median τeff from ≃2 at z = 2.7 to τeff≳5 at z > 3, and a scatter in τeff 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 τeff 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 ΓHeII of a fluctuating UV background from the median τeff, finding a factor ≃5 decrease in ΓHeII between z ≃ 2.6 and z ≃ 3.1. At z ≃ 3.1, ΓHeII=[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.
© 2019. The American Astronomical Society. All rights reserved.
dark ages, reionization, first stars - diffuse radiation - intergalactic medium - quasars: absorption lines
Status at CDS
Large table(s) of objects being ingested in VizieR. // Associated data (images, light curves, etc...) being ingested in VizieR.
View the reference in ADS
To bookmark this query, right click on this link: simbad:2019ApJ...875..111W and select 'bookmark this link' or equivalent in the popup menu