2010ApJ...721.1448S

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 ∼ 1, with GALEX grism spectrograph observations of 50 quasars with 0.8 < z_em < 1.3, and (2) extend coverage to z ∼ 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]^γ with n_3.5= 2.80±0.33 and γ = 1.94^+0.36_–0.32 for the entire range 0 < z < 6. We have also reanalyzed the evolution of damped Lyα 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 logH i 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 Ω_DLA= 9x10^–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 ∼ 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.