High Redshift Lyman Limit and Damped Lyman-Alpha Absorbers

Abstract

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-a 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 logN(HI)≥ 17.5, over the redshift range 0.01 < z < 5. The main results are: (i) Lyman limit systems: The data are well fit by a power law N(z) = N 0(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 N 0 = 0.27 with >99.7% confidence limits for γ of 0.82 & 2.37. (ii) Damped Lyα 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 Lya 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, Ω g. 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. KeywordsColumn DensityHigh RedshiftRoyal Greenwich ObservatoryRedshift RangeMadingley RoadThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.