Metal Enrichment, Dust and Star Formation in High-redshift Galaxies

Abstract

QSO absorption systems exhibiting a damped Lyoc line are thought to arise in the high·redshift progenitors of present-day disk galaxies. In order to clarify the evolutionary status of these absorbers and interpret the clues they provide to the process of galaxy formation in general, we have begun a programme of spectroscopic observations targeted at measuring their element abundances, dust concentration and star·formation rates. Abundance studies in the local interstellar medium (ISM) have identified Zn as a highly convenient species for surveying the overall level of metallicity of the damped systems, while a qualitative indication of the dust content can be obtained by consideration of the Zn·to·Cr ratio since, unlike Zn, Cr is readily bound to interstellar grains. In some favourable cases, higher·resolution observations of selected lines can provide additional information on abundance ratios of individual elements. Lastly, searches for weak emission in the black core of the damped Lyoc absorption line have so far provided the most stringent limits on the Lyoc luminosity of these supposedly young galaxies, and in one case led to a positive detection. The results obtained so far suggest that the damped Lyoc galaxies are indeed at an early phase of chemical evolution, with metal abundances 1 to 2 orders of magnitude below solar and with perhaps only 1/10 of the dust content of the local ISM. Lyoc fluxes are well below the predictions of many theoretical models, and star-formation rates < 10 MGl yr-1 seem to be typical. These properties are similar to those of present·day H II galaxies of which the Lyoc systems may therefore be high·redshift examples.