Journal
ASTROPHYSICAL JOURNAL
Volume 875, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/ab0fa1
Keywords
dark ages, reionization, first stars; diffuse radiation; intergalactic medium; quasars: absorption lines
Categories
Funding
- NASA from the Space Telescope Science Institute [50 OR 1317, 50 OR 1512, HST-AR-15014]
- NASA [NAS5-26555]
- Deutsches Zentrum fur Luft- und Raumfahrt (DLR) [50 OR 1317, 50 OR 1512]
- Alfred P. Sloan Foundation
- National Science Foundation
- U.S. Department of Energy Office of Science
- University of Arizona
- Brazilian Participation Group
- Brookhaven National Laboratory
- Carnegie Mellon University
- University of Florida
- French Participation Group
- German Participation Group
- Harvard University
- Instituto de Astrofisica de Canarias
- Michigan State/Notre Dame/JINA Participation Group
- Johns Hopkins University
- Lawrence Berkeley National Laboratory
- Max Planck Institute for Astrophysics
- Max Planck Institute for Extraterrestrial Physics
- New Mexico State University
- New York University
- Ohio State University
- Pennsylvania State University
- University of Portsmouth
- Princeton University
- Spanish Participation Group
- University of Tokyo
- University of Utah
- Vanderbilt University
- University of Virginia
- University of Washington
- Yale University
- National Aeronautics and Space Administration through the Planetary Science Division of the NASA Science Mission Directorate [NNX08AR22G]
- National Science Foundation [AST-1238877]
- National Aeronautics and Space Administration
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We present measurements of the large-scale (approximate to 40 comoving Mpc) effective optical depth of He II Ly alpha absorption, tau(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 tau(eff) measurements in six signal-to-noise ratio greater than or similar to 3 He II sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of tau(eff) in 24 He II sightlines. We confirm an increase of the median tau(eff) from similar or equal to 2 at z = 2.7 to tau(eff) greater than or similar to 5 at z > 3, and a scatter in tau(eff) that increases with redshift. The z > 3.5 He II absorption is predominantly saturated, but isolated narrow (Delta v < 650 kms(-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 teff 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 Gamma(He) (II) of a fluctuating UV background from the median tau(eff), finding a factor similar or equal to 5 decrease in Gamma(He) (II) between z similar or equal to 2.6 and z similar or equal to 3.1. At z similar or equal to 3.1, Gamma(He II) = [9.1(-1.2)(+1.1) (stat) (+2.4)(-3.4) (sys)] x 10(-16) s(-1) corresponds to a median He II fraction of similar or equal to 2.5%, indicating that our data probe the tail end of He II reionization.
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