Abstract
We study the evolution of the ionization state of the intergalactic medium (IGM) at the end of the reionization epoch using moderate-resolution spectra of a sample of 19 quasars at 5.74 5.7: the GP optical depth evolution changes from τ ~ (1 + z)4.3 to (1 + z)11, and the average length of dark gaps with τ > 3.5 increases from 80 comoving Mpc. The dispersion of IGM properties along different lines of sight also increases rapidly, implying fluctuations by a factor of 4 in the UV background at z > 6, when the mean free path of UV photons is comparable to the correlation length of the star-forming galaxies that are thought to have caused reionization. The mean length of dark gaps shows the most dramatic increase at z ~ 6, as well as the largest line-of-sight variations. We suggest using dark gap statistics as a powerful probe of the ionization state of the IGM at yet higher redshift. The sizes of H II regions around luminous quasars decrease rapidly toward higher redshift, suggesting that the neutral fraction of the IGM has increased by a factor of 10 from z = 5.7 to 6.4, consistent with the value derived from the GP optical depth. The mass-averaged neutral fraction is 1%-4% at z ~ 6.2 based on the GP optical depth and H II region size measurements. The observations suggest that z ~ 6 is the end of the overlapping stage of reionization and are inconsistent with a mostly neutral IGM at z ~ 6, as indicated by the finite length of the dark absorption gaps.
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