THE INCIDENCE OF DAMPED LY-ALPHA SYSTEMS IN THE REDSHIFT INTERVAL O-LESS-THAN-Z-LESS-THAN-4

The determination of the incidence and column density distribution of damped Ly alpha 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 similar to 62% of the age of the universe for q(0) = 0 (77% for q(0) = 0.5). However, because damped Ly alpha 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 Ly alpha systems studied so far are found to have corresponding Mg Ir metal-line absorption. For redshifts z < 1.65 we have used available UV data to study the Ly alpha absorption line corresponding to 43 Mg Pi systems. After accounting for possible biases, we find that at a mean redshift of (z) = 0.8 the number of damped Ly alpha absorption lines with neutral hydrogen column density N(H I) greater than or equal to 2 x 10(20) cm(-2) per unit redshift is n(DLya)(z = 0.8) less than or equal to 0.12. A similar analysis based on studying the Ly alpha line in Lyman-limit systems is consistent with this result. This limit on the observed number density at z approximate to 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 approximate to 4 to the present epoch: n(DLya)(z) = (0.015 +/- 0.004) (1 + z)(2.27) +/- (0.25). The power-law index of 2.27 +/- 0.25 indicates that damped Ly alpha absorbers undergo evolution, either in size or in number density, from a redshift of about 4 to the present epoch.