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

Abstract

Abstract. We present metal abundances in 15 damped Lyα 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 Lyα 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.

The dust obscuration bias in damped Lyman αsystems

“…This large difference is probably be due to the fact that the weighted metallicity is extremely sensitive to the number of high column-density DLAs included in the average, which are the DLAs more affected by obscuration. Our results are marginally consistent with the mean weighted metallicity [ (Zn/H) DLA ] = −0.88 ± 0.21 dex obtained from the CORALS metallicity survey (Akerman et al 2005).…”

THE PRIMORDIAL DEUTERIUM ABUNDANCE OF THE MOST METAL-POOR DAMPED Lyα SYSTEM∗

“…Another possibility is that deuterium may be preferentially depleted onto dust grains (Jura 1982;Draine 2004Draine , 2006 (Pettini et al 1997;Vladilo 2004;Akerman et al 2005), and very low-metallicity DLAs are not expected to harbor a significant amount of dust (see Murphy & Bernet 2016, and references therein). Ultimately, this issue will be clarified by extending the number of precision measures of D/H over a wider range of metallicity than covered by the present sample.…”

Silicon depletion in damped Ly αsystems

“…Starting from this study, it became clear that it is unsafe to trace the metallicity 3 of DLA systems using refractory elements, such as iron. For this reason, zinc has become the first choice in studies of DLA metallicity, yielding a typical value [Zn/H] −1.1 dex (Pettini et al 1994;Akerman et al 2005), with a large dispersion at each redshift and a gradual rise with cosmic time (e.g. Vladilo et al 2000;Prochaska et al 2001).…”