Hubble Space Telescope Observations of Element Abundances in Low-Redshift Damped Lyα Galaxies and Implications for the Global Metallicity-Redshift Relation

Abstract

Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value ~1/30 solar at z~3 to a nearly solar value at z=0. In the absence of any selection effects, the damped Lyα absorbers (DLAs) in quasar spectra are expected to show such a rise in global metallicity. However, it has been difficult to determine whether or not DLAs show this effect, primarily because of the very small number of DLA metallicity measurements at low redshifts. In an attempt to put tighter constraints on the low-redshift end of the DLA metallicity-redshift relation, we have observed Zn II and Cr II lines in four DLAs at 0.09<z<0.52, using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). These observations have provided the first constraints on Zn abundances in DLAs with z<0.4. In all three DLAs for which our observations offer meaningful constraints on the metallicity, the data suggest that the metallicities are much lower than the solar value. These results are consistent with recent imaging studies indicating that these DLAs may be associated with dwarf or low surface brightness galaxies. We combine our results with higher redshift data from the literature to estimate the global mean metallicity-redshift relation for DLAs. We find that the global mean metallicity shows at most a slow increase with decreasing redshift. For the redshift range 0.09<z<3.90, the slope of the exponential fit to the binned N_HI-weighted mean Zn metallicity versus redshift relation is -0.18+/-0.06 counting Zn limits as detections, -0.22+/-0.08 counting Zn limits as zeros, and -0.23+/-0.06 using constraints on metallicity from other elements instead of the Zn limits. The corresponding estimates of the z=0 intercept of the metallicity-redshift relation are -0.74+/-0.15, -0.75+/-0.18, and -0.71+/-0.13, respectively. Roughly similar results are obtained if survival analysis or an unbinned N_HI-weighted nonlinear χ² approach is used. Thus, the N_HI-weighted mean metallicity of DLAs does not appear to rise up to solar or near-solar values at z=0. This weak evolution could be explained by the fact that our absorption-selected sample seems to be dominated by dwarf or low surface brightness galaxies. This suggests that current DLA samples, especially those at low redshifts, could be biased against more enriched galaxies because the latter may cause higher dust obscuration of the background quasars.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.