The Incidence of Damped LY alpha Systems in the Redshift Interval 0 < Z < 4

Abstract

The determination of the incidence and column density distribution of damped Lyα systems from QSO absorption-line surveys in the redshift interval 0 < z < 1.65 is crucial for constraining the evolution of neutral gas in galaxies. This redshift interval corresponds to a time interval of ∼62% of the age of the universe for q₀ = 0 (77% for q₀ = 0.5). However, because damped Lyα systems are relatively rare in spectra, a sensitive UV spectroscopic survey would consume considerable telescope time. We have therefore undertaken a new approach that takes advantage of the fact that all damped Lyα systems studied so far are found to have corresponding Mg II metal-line absorption. For redshifts z < 1.65 we have used available UV data to study the Lyα absorption line corresponding to 43 Mg II systems. After accounting for possible biases, we find that at a mean redshift of <z> = 0.8 the number of damped Lyα absorption lines with neutral hydrogen column density N(H I) ≥ 2 × 10²⁰ cm^-2 per unit redshift is n_DLyα(z = 0.8) ≤ 0.12. A similar analysis based on studying the Lyα line in Lyman-limit systems is consistent with this result.

This limit on the observed number density at z ≈ 0.8 is consistent with the trend at higher redshift as well as the number density at the current epoch inferred from H I cross sections of nearby disk galaxies. The result that n(z) in the redshift interval 0 < z < 1.65 conforms to a trend established by three different observing techniques over three redshift regimes provides evidence that this trend actually traces the evolution of neutral gas in galaxies. The data at z > 1.65 combined with the constraint from nearby galaxies at redshift z = 0 result in a parameterization that is valid from redshift z ≈ 4 to the present epoch: n_DLyα(z) = (0.015±0.004) (1 + z)^2.27±0.25. The power-law index of 2.27±0.25 indicates that damped Lyα absorbers undergo evolution, either in size or in number density, from a redshift of about 4 to the present epoch.