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

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 < z_q < 6.4, enabling the first proximity-effect measurements of the UVB at z > 5. The metagalactic hydrogen ionization rate, Γ_bkg, 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 Γ_bkg. 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 Γ_bkgwere also investigated, but found to be small at z > 5. We find a decline in Γ_bkg with redshift, from log(Γ_bkg) =-12.15 ± 0.16 at z∼ 5 to log(Γ_bkg) =-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 Γ_bkg 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 Γ_bkg and the mean free path. A part of the observations were made at the W. M. Keck Observatory which is operated as a scientific partnership between the California Institute of Technology and the University of California; it was made possible by the generous support of the W. M. Keck Foundation. This paper also includes data gathered with the 6.5-m Magellan Telescopes located at Las Campanas Observatory, Chile.