Constraining the Evolution of the Ionizing Background and the Epoch of Reionization withz ~ 6 Quasars. II. A Sample of 19 Quasars

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 nineteen quasars at 5. 74 5. 7: the GP optical depth evolution changes from tausub GPsup eff approx (1 + z) sup 4. 3 to (1 + z) sup {approx> 11}, and the average length of dark gaps with tau > 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 approx> 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 approx 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 HII regions around luminous quasars decrease rapidly towards higher redshift, suggesting that the neutral fraction of the IGM has increased by a factor of approx> 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 approx 6. 2 based on the GP optical depth and HII region size measurements. The observations suggest that z approx 6 is the end of the overlapping stage of reionization, and are inconsistent with a mostly neutral IGM at z approx 6, as indicated by the finite length of dark absorption gaps.