Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of [ITAL][CLC]z[/CLC][/ITAL] ∼ 6 Quasars

We study the process of cosmic reionization and estimate the ionizingbackground in the IGM using the Lyman series absorption in the spectra of thefour quasars at 5.710^{-3} at z~6. At this redshift, the mass-averaged neutralhydrogen fraction is larger than 1%; the mildly overdense regions (delta > 3)are still mostly neutral and the comoving mean free path of ionizing photons isshorter than 8 Mpc. Comparison with simulations of cosmological reionizationshows that the observed properties of the IGM at z~6 are typical of those inthe era at the end of the overlap stage of reionization when the individual HIIregions merge. Thus, z~6 marks the end of the reionization epoch. The redshiftof reionization constrains the small scale power of the mass densityfluctuations and the star forming efficiency of the first generation ofobjects.Comment: AJ accepted, 27 pages; minor change