Approaching Reionization: The Evolution of the L[CLC]y[/CLC]α Forest from [CLC][ITAL]z[/ITAL][/CLC] = 4 to [CLC][ITAL]z[/ITAL][/CLC] = 6

We analyze the Ly alpha forest properties of a sample of 15 high redshiftquasars lying between z = 4.42 and z = 5.75, using high signal-to-noise spectraobtained with ESI on the Keck II 10 m telescope. The distribution oftransmissions in the Ly alpha region in this redshift range is shown to beconsistent with that found in lambda cold dark matter simulations with asmoothly evolving ionization rate as a function of redshift. The extrapolationof the ionizing flux to z = 6.05 lies a factor of two higher than a 2 sigmaupper limit placed by Cen & McDonald (2001) at this redshift, based on theBecker et al. (2001) spectra of the z = 6.28 quasar SDSS 1030+0524. However,the data are also consistent with models in which there is substantialvariation of the ionization parameter about the mean value, and in this case,dark gaps such as those seen by Becker et al. become much more likely. Weconclude that further data are needed before we can be sure that we have seenthe epoch of reionization. We also summarize the damped Ly alpha systems seenin these quasar lines of sight and measure their metallicities and kinematicproperties. We argue that the mean DLA metallicity has dropped substantially byz = 5 compared with its value at z < 4.Comment: 17 pages, including 13 figures. Accepted for publication in Astronomical Journal (May 2002