Astron. Astrophys. 341, 709-724 (1999)

1. Introduction

Abundances of various heavy elements in Damped Ly (DLA ) absorbers are being determined since many years now, first mainly in the redshift range where the DLA lines are accessible from the ground. Abundances have been determined from curve of growth analysis e.g. of the low ionisation lines of ZnII, CrII, FeII, SiII, and many others, which are found associated with the DLA line. The HST key project Quasar Absorption Lines (Bahcall et al. 1996, 1993) extended the possible range of DLA detections towards lower redshifts. In particular three observational facts still keep challenging our understanding of the nature of DLA systems.

• The number of low redshift DLA systems detected is much smaller than expected.

• The redshift evolution of heavy element abundances in DLA systems is very weak, in particular if compared to the strong redshift evolution of the narrow high ionisation CIV QSO absorption systems.

• The scatter of abundances observed among various DLA systems at any given redshift is very large.

In 1995 we presented a first comparison of our chemical and cosmological galaxy evolution models for spiral galaxies of various types with DLA abundances (Fritze - v. Alvensleben & Fricke 1995b, see also Fritze - v. Alvensleben 1995a). At that time, however, observational abundance determinations were not very precise yet, giving lower and upper limits only in many cases and we felt somewhat uncomfortable using stellar yields calculated for solar metallicity stars to compare with high redshift DLAs which typically have low metallicities .

Meanwhile, the situation has improved considerably. KECK HIRES, HST GHRS, and WHT spectra give completely resolved absorption line profiles for a large number of lines in many DLAs. In these cases the apparent optical depth method, which does not require any assumption about (the functional form of) the velocity distribution in the absorbing gas, allows for precise abundance determinations (e.g. Pettini et al. 1994, Prochaska & Wolfe 1996, Lu et al. 1996). This method works well even for kinematically complex multi-component profiles. It also allows to correct for saturated lines, in which case still, however, only lower limits for the respective element abundances can be obtained.

On the theoretical side, stellar yields for a set of different metallicities from Z = 0 to have become available (Woosley & Weaver 1995, van den Hoek & Groenewegen 1997, Marigo et al. 1998, Portinari et al. 1998) which we now use in our modeling. We have developed a method to model the chemical evolution of ISM abundances in galaxies in a chemically consistent way, very much in parallel to the chemically consistent treatment of the photometric (Einsel et al. 1995, Fritze - v. Alvensleben et al. 1996, Möller et al. 1997) and spectral (Möller et al. 1998) evolution. For a galaxy, which always is a composite system in terms of stellar metallicities and ages we follow the chemical evolution of successive generations of stars using yields and stellar lifetimes appropriate for their respective initial metallicities.

We use a set of star formation histories (SFH s) appropriate for spiral galaxies of various types that provide a successful description not only of the detailed spectral properties of the respective nearby template galaxies in the optical, their average colors from U through K, their emission line properties but also of their redshift evolution back to as far as accessible via type-dependent redshift surveys (cf. Möller et al. 1996 and 1998). We show that with these SFHs the chemically consistent chemical and cosmological evolution models give good agreement of the model abundances at with observed HII region abundances of nearby galaxies. We thus expect our models to also be able to describe the redshift evolution of the ISM abundances in spiral or proto-spiral galaxies and thus to provide a tool to bridge the gap between DLA absorbers at high redshift and the local galaxy population.

We present our models, their basic parameters, and the input physics we use in Sect. 2. All available DLA abundance data are compiled and described in Sect. 3 where we also discuss their various degrees of reliability. In Sect. 4, we present a detailed comparison of the redshift evolution of various element abundances (Fe, Si, Zn, Cr, Ni, S, Al, Mn) as given by our models for various spiral types with all the available data for DLAs. Results are discussed as to our understanding of the weak redshift evolution of observed DLA abundances, the scatter they show at any given redshift, the question as to the nature of the absorber galaxy/protogalaxy population as well as to the importance of the chemically consistent approach. In Sect. 5 we use the comparison between models and observations to discuss the properties of the DLA absorbing galaxy population and their redshift evolution, derive some implications and present predictions for optical identifications of DLA galaxies. We summarize our main results in Sect. 6.

© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998
helpdesk.link@springer.de