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 similar to 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, Gamma(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 Ly alpha forest, and from this we make a measurement of Gamma(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 similar to 5, while uncertainties in the mean transmitted Ly alpha flux are responsible for the largest bias at z similar to 6. The impacts of large-scale overdensities and Lyman limit systems on Gamma(bkg)were also investigated, but found to be small at z > 5. We find a decline in Gamma(bkg) with redshift, from log(Gamma(bkg)) = -12.15 +/- 0.16 at z similar to 5 to log(Gamma(bkg)) = -12.84 +/- 0.18 at z similar to 6 (1 Sigma 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 similar to 4 and 6. The decline of Gamma(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 similar to 5 to 6, albeit with significant uncertainty due to the measurement errors in both Gamma(bkg) and the mean free path.