Approaching Reionization: The Evolution of the Lyman Alpha Forest from Redshifts Four to Six

We analyze the Ly alpha forest properties of a sample of 15 high redshift quasars lying between z = 4.42 and z = 5.75, using high signal-to-noise spectra obtained with ESI on the Keck II 10 m telescope. The distribution of transmissions in the Ly alpha region in this redshift range is shown to be consistent with that found in lambda cold dark matter simulations with a smoothly evolving ionization rate as a function of redshift. The extrapolation of the ionizing flux to z = 6.05 lies a factor of two higher than a 2 sigma upper limit placed by Cen & McDonald (2001) at this redshift, based on the Becker 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 substantial variation 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. We conclude that further data are needed before we can be sure that we have seen the epoch of reionization. We also summarize the damped Ly alpha systems seen in these quasar lines of sight and measure their metallicities and kinematic properties. We argue that the mean DLA metallicity has dropped substantially by z = 5 compared with its value at z < 4