High Redshift Lyman Limit and Damped Lyman-Alpha Absorbers

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We have obtained high signal:to:noise optical spectroscopy at 5Å resolution of 27 quasars from the APM z>4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-α absorbers. These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with (HI)≥17.5, over the redshift range 0.01 < z < 5. The main results are: itemize Lyman limit systems: The data are well fit by a power law N(z) = N₀(1 + z)γ for the number density per unit redshift. For the first time intrinsic evolution is detected in the product of the absorption cross-section and comoving spatial number density for an Ω= 1 Universe. We find γ= 1.55 (γ= 0.5 for no evolution) and N₀ = 0.27 with >99.7% confidence limits for γ of 0.82 & 2.37. Damped systems: The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. Eleven candidate and three confirmed damped Lyα absorption systems, have been identified in the APM QSO spectra covering the redshift range 2.8≤ z ≤ 4.4 (11 with z>3.5). Combining the APM survey confirmed and candidate damped absorbers with previous surveys, we find evidence for a turnover at z 3 or a flattening at z 2 in the cosmological mass density of neutral gas, Ωg. itemize The Lyman limit survey results are published in Storrie-Lombardi, et al., 1994, ApJ, 427, L13. Here we describe the results for the DLA population of absorbers.