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Measurements of the ultraviolet background at 4.6 < z < 6.4 using the quasar proximity effect

Measurements of the ultraviolet background at 4.6 < z < 6.4 using the quasar proximity effect Thumbnail


We present measurements of the ionizing ultraviolet background (UVB) at z∼ 5–6 using the quasar proximity effect. The 15 quasars in our sample cover the range 4.6 < zq < 6.4, enabling the first proximity-effect measurements of the UVB at z > 5. The metagalactic hydrogen ionization rate, Γbkg, was determined by modelling the combined ionization field from the quasar and the UVB in the proximity zone on a pixel-by-pixel basis. The optical depths in the spectra were corrected for the expected effect of the quasar until the mean flux in the proximity region equalled that in the average Lyα forest, and from this we make a measurement of Γbkg. A number of systematic effects were tested using synthetic spectra. Noise in the flux was found to be the largest source of bias at z∼ 5, while uncertainties in the mean transmitted Lyα flux are responsible for the largest bias at z∼ 6. The impacts of large-scale overdensities and Lyman limit systems on Γbkgwere also investigated, but found to be small at z > 5. We find a decline in Γbkg with redshift, from log(Γbkg) =−12.15 ± 0.16 at z∼ 5 to log(Γbkg) =−12.84 ± 0.18 at z∼ 6 (1σ errors). Compared to UVB measurements at lower redshifts, our measurements suggest a drop of a factor of 5 in the H I photoionization rate between z∼ 4 and 6. The decline of Γbkg appears to be gradual, and we find no evidence for a sudden change in the UVB at any redshift that would indicate a rapid change in the attenuation length of ionizing photons. Combined with recent measurements of the evolution of the mean free path of ionizing photons, our results imply a decline in the emissivity of ionizing photons by roughly a factor of 2 from z∼ 5 to 6, albeit with significant uncertainty due to the measurement errors in both Γbkg and the mean free path.

Journal Article Type Article
Acceptance Date Nov 23, 2010
Online Publication Date Apr 8, 2011
Publication Date 2011-04
Deposit Date Dec 27, 2022
Publicly Available Date Jan 20, 2023
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 412
Issue 4
Pages 2543-2562
Public URL
Publisher URL


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