Zinc as a tracer of metallicity evolution of Damped Ly α systems

Zinc is a good indicator of metallicity in Damped Lyman α (DLA) systems because it is almost unaffected by dust depletion. However, the use of zinc as a tracer of metallicity evolution has been hampered by the difficulty of detecting the Zn II resonance lines at high redshift. The measurement of zinc abundance in a DLA system at zabs > 3 obtained by means of the UVES spectrograph at the VLT prompted us to re-analyse the full sample of zinc abundances present in the literature to search for a metallicity-redshift relation in DLA systems. The study of the metallicities of individual systems shows evidence for an anti-correlation between [Zn/H] and redshift supported by different types of statistical tools. The zinc metallicity decreases by −0.3 ± 0.1 dex per unit redshift interval in the range 0.5 ∼ zabs < ∼ 3.5. This rate is in good agreement with that found by Savaglio, Panagia & Stiavelli (2000) in their recent study of DLA abundances corrected for dust depletion. The present result does not require a knowledge of the dust depletion pattern(s) in DLA systems. On the other hand, the analysis of the column-density weighted metallicity of the sample, < Z >, does not show a clear evidence for redshift evolution, consistent with previous studies of zinc abundances. We propose that the apparent lack of evolution of < Z > is due to the combination of selection bias effects together with the extreme sensitivity of < Z > to low-number statistics. Subject headings: cosmology: observations — galaxies: abundances — galaxies: evolution — quasars: absorption lines