Metallicity evolution of damped Lyα galaxies

We have reanalyzed the existing data on zinc abundances in damped Lyalpha (DLA) absorbers to investigate whether their mean metallicity evolves with time. Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value of similar to1/30 solar at z similar to 3 to a nearly solar value at z similar to 0. However, several previous analyses have suggested that there is little or no evolution in the global metallicity of DLAs. The main problem is that the effective number of systems that dominate the N(H I)-weighted mean metallicity is very small. We have used a variety of statistical techniques to quantify the global metallicity-redshift relation and its uncertainties, taking into account both measurement and sampling errors. Three new features of our analysis are (1) an unbinned N(H I)-weighted nonlinear chi(2) fit to an exponential relation, (2) survival analysis to treat the large number of limits in the existing data, and (3) a comparison of the data with several models of cosmic chemical evolution based on an unbinned N(H I)-weighted chi(2). We find that a wider range of evolutionary rates is allowed by the present data than claimed in previous studies. The slope of the exponential fit to the N(H I)-weighted mean Zn metallicity versus redshift relation is -0.20 +/- 0.11 counting limits as detections and -0.27 +/- 0.12 counting limits as zeros. Similar results are also obtained if the data are binned in redshift and if survival analysis is used. These slopes are marginally consistent with no evolution but are also consistent with the rates predicted by several models of cosmic chemical evolution (e.g., slopes of -0.61 to -0.25 for the models of Pei & Fall, Malaney & Chaboyer, and Pei et al.). The chi(2) values obtained for most of these models are somewhat worse than that for the exponential model because the models lie above the observed data points, but still suggest that the present DLA data could indicate some evolution of the metallicity with redshift. Finally, we outline some future measurements necessary to improve the statistics of the global metallicity-redshift relation.