THE EVOLUTION OF LYMAN LIMIT ABSORPTION SYSTEMS TO REDSHIFT SIX

We have measured the redshift evolution of the density of Lyman limit systems (LLSs) in the intergalactic medium over the redshift range 0 < z < 6. We have used two new quasar samples to (1) improve coverage at z similar to 1, with GALEX grism spectrograph observations of 50 quasars with 0.8 < z(em) < 1.3, and (2) extend coverage to z similar to 6, with Keck ESI spectra of 25 quasars with 4.17 < z(em) < 5.99. Using these samples together with published data, we find that the number density of LLS per unit redshift, n(z), can be well fit by a simple evolution of the form n(z) = n(3.5)[(1 + z)/ 4.5](gamma) with = n(3.5) = 2.80 +/- 0.33 and gamma = 1.94(-0.32)(+0.36) for the entire range 0 < z < 6. We have also reanalyzed the evolution of damped Lya systems (DLAs) in the redshift range 4 < z < 5 using our high-redshift quasar sample. We find a total of 17 DLAs and sub-DLAs, which we have analyzed in combination with published data. The DLAs with logHi column density > 20.3 show the same redshift evolution as the LLS. When combined with previous results, our DLA sample is also consistent with a constant Omega(DLA) = 9 x 10(-4) from z = 2 to z = 5. We have used the LLS number density evolution to compute the evolution in the mean free path (mfp) of ionizing photons. We find a smooth evolution to z similar to 6, very similar in shape to that of Madau et al. but about a factor of two higher. Recent theoretical models roughly match to the z < 6 data but diverge from the measured power law at z > 6 in different ways, cautioning against extrapolating the fit to the mfp outside the measured redshift range.