The Age-Metallicity Relation of the Universe in Neutral Gas: The First 100 Damped Lyα Systems

We discuss accurate metallicity measurements for 125 damped Lyα (DLA) systems at 0.5<z<5, including ~50 new measurements from our recently published Echellette Spectrograph and Imager surveys. This data set is analyzed to determine the age-metallicity relation of neutral gas in the universe. Contrary to previous analyses, this sample shows statistically significant evolution in the cosmic mean metallicity. The best linear fit for metallicity evolution is -0.26+/-0.07 dex per unit redshift. The DLA systems continue to maintain a floor in metallicity of ~1/700 solar, independent of observational effects. This metallicity threshold limits the prevalence of primordial gas in high-redshift galaxies and stresses the correspondence between damped systems and star formation. Finally, we comment on an apparent ``missing metals problem:'' the mean metallicity of the damped systems is ~10 times lower than the value expected from their inferred star formation history. This problem is evident in current theoretical treatments of galaxy formation and may indicate a serious flaw in our understanding of the interplay between star formation and metal enrichment.