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New measurements of the ionizing ultraviolet background over 2 < z < 5 and implications for hydrogen reionization

Becker, George D.; Bolton, James S.


George D. Becker


We present new measurements of the intensity of the ionizing ultraviolet background and the global emissivity of ionizing photons over 2 < z < 5. Our results are based on a suite of updated measurements of physical properties of the high-redshift intergalactic medium (IGM), including gas temperatures and the opacity of the IGM to Lyα and ionizing photons. Consistent with previous works, we find a relatively flat hydrogen photoionization rate over 2 < z < 5, although our measurements are roughly a factor of 2 higher than the 2008 values of Faucher-Giguère et al., due primarily to our lower gas temperatures. The ionizing emissivity we derive is also generally higher than other recent estimates due to a combination of lower gas temperatures, higher ionizing opacity and an accounting of cosmological radiative transfer effects. We find evidence that the emissivity increases from z ∼ 3 to 5, reaching ∼5 ionizing photons per atom per Gyr at z = 4.75 for realistic galaxy spectra. We further find that galaxies must dominate the emissivity near 1 Ryd at z ≥ 4, and possibly at all redshifts z ≥ 2.4. Our results suggest that the globally averaged ionizing ‘efficiency’ of star-forming galaxies increases substantially with redshift over 3.2 ≤ z ≤ 4.75. This trend is consistent with the conclusion often drawn from reionization models that the ionizing efficiency of galaxies must be higher during reionization in order for galaxies to reionize the IGM by z = 6. Our emissivity values at z ∼ 5 suggest that ionizing photons may have been a factor of 2 more abundant during the final stages of reionization than previously indicated. The evolution of the ionizing emissivity over 2 < z < 5 suggests, moreover, that the steep decline in the photoionization rate from z ∼ 5 to 6 may indicate a rapid evolution in the mean free path at z > 5.


Becker, G. D., & Bolton, J. S. (2013). New measurements of the ionizing ultraviolet background over 2 < z < 5 and implications for hydrogen reionization. Monthly Notices of the Royal Astronomical Society, 436(2), 1023-1039.

Journal Article Type Article
Acceptance Date Aug 22, 2013
Online Publication Date Oct 16, 2013
Publication Date Dec 1, 2013
Deposit Date Dec 27, 2022
Publicly Available Date Jan 20, 2023
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 0035-8711
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 436
Issue 2
Pages 1023-1039
Keywords Space and Planetary Science; Astronomy and Astrophysics
Public URL
Publisher URL
Additional Information This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


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