Zn and Cr abundances in damped Lyman alpha systems from the CORALS survey

We present metal abundances in 15 damped Ly alpha systems (DLAs) from the Complete Optical and Radio Absorption Line System ( CORALS) survey, designed to be free from any biasing effects due to extinction of QSOs by dust in intervening absorbers. It has long been suggested that such biasing may explain differences in metallicity between damped Lya systems and coeval luminous galaxies, and between model predictions and observations. We use our measured zinc and chromium abundances ( combined with those for five more CORALS DLAs from the literature, giving us a very nearly complete sample) to test whether the metallicity and degree of dust depletion in CORALS DLAs are significantly different from those of existing, larger, samples of DLAs drawn from magnitude limited, optical surveys. We find that the column density weighted metallicity of CORALS DLAs, [<( Zn/H)(DLA)>] = -0.88 +/- 0.21 in the redshift interval 1.86 < z(abs) < 3.45, is only marginally higher than that of a control sample from the by Kulkarni et al., [<( Zn/H)(DLA)>] = -1.09 +/- 0.10. With the present limited statistics this difference is not highly significant. Furthermore, we find no evidence for increased dust depletions in CORALS DLAs - their [Cr/Zn] ratios conform to the known trend of increasing depletion ( decreasing [Cr/Zn]) with increasing metallicity, and we have encountered no cases where Cr is as depleted as in local cold interstellar clouds. These results, when combined with the earlier findings of the CORALS survey reported by Ellison et al. in 2001, make it difficult to invoke a dust-induced bias to explain the generally low level of chemical evolution exhibited by most DLAs. Rather, they indicate that large scale optical QSO surveys give a fair census of the population of high redshift absorbers.