The evolution of Ω_H iand the epoch of formation of damped Lyman α absorbers

We present a study of the evolution of the column density distribution, f(N, z), and total neutral hydrogen mass in high-column density quasar absorbers using data from a recent high-redshift survey for damped Lyman-α (DLA) and Lyman limit system (LLS) absorbers. The observed number of LLS (N(HI)> 1.6 × 10 17 atom cm −2) is used to constrain f(N, z) below the classical DLA Wolfe et al. (1986) definition of 2 × 10 20 atom cm −2. We find the evolution of the number density of LLS is consistent with our previous work but steeper than previously published work of other authors. At z=5, the number density of Lyman-limit systems per unit redshift is ∼5, implying that these systems are a major source of UV opacity in the high redshift Universe. The joint LLS-DLA analysis shows unambiguously that f(N, z) deviates significantly from a single power law and that a Γ-law distribution of the form f(N, z) = (f∗/N∗)(N/N∗) −β exp(−N/N∗) provides a better description of the observations. These results are used to determine the amount of neutral gas contained in DLAs and in systems with lower column density. Whilst in the redshift range 2 to