1 Introduction

Since the discovery that intermediate redshift ( $z \sim 0.3$) clusters exhibit a relative over-abundance of blue member galaxies compared with the local population (Butcher & Oemler 1987), extensive work has investigated the evolution of the cluster environment and compared it with the field population (e.g. Poggianti et al. 1999 and references therein). From these recent spectroscopic studies of $z \sim 0.4$clusters, it has been established that star formation is generally suppressed in these rich environments, but that post-starburst (E + A) galaxies make up $\sim$20% of the cluster population (Dressler et al. 1999). In addition, there appears to be a radial star formation rate gradient in clusters that is independent of the morphology-density relation, such that galaxies with the most recent star formation episodes occur farther out (Balogh et al. 1999). Once accreted into the cluster, active star formation seems to be swiftly quenched (Dressler et al. 1999) and continues at a relatively low rate (e.g. Couch et al. 2001).

Figure 1: Profile fit to the double DLA towards B2314-409. The two absorbers have $\log$ N(H I) = 20.9 at $z_{\rm abs}$ = 1.8573 (DLA A) and $\log$ N(H I) = 20.1 at $z_{\rm abs}$ = 1.8745 (DLA B).

However, these informative surveys have not been extended beyond $z \sim 1$ due to the lack of good quality spectroscopic data at these redshifts, although wide field surveys at X-ray, optical and near-IR wavelengths have detected clusters out to $z \sim 1.3$ (e.g. Rosati et al. 1998). At earlier epochs, the study of Lyman break galaxies (LBGs) has permitted the discovery of large galaxy overdensities at $z \sim 3$ (Steidel et al. 1998). However, it is important to bear in mind that being such biased tracers of matter, LBGs are very different from typical cluster galaxies at low redshifts. We are left, therefore, with a significant gap in our knowledge of groups and clusters of galaxies between . In particular, this leaves open many issues involving the early evolution of galaxy groups. For example, at what stage does the environment start to affect the star formation of the individual galaxies and is the activity boosted prior to being truncated?

One of the most promising techniques for detecting representative galaxies at is using QSO absorption lines, although the possibilities for studying clusters of absorbing galaxies is more limited. Nonetheless, some observations of high column density absorbers, in particular damped Lyman alpha systems (DLAs), along multiple lines of sight have been supplemented with Lyman break and narrow band Lyman $\alpha$ imaging to show that DLAs can reside in galaxy concentrations out to $z \sim 3.5$(e.g. Francis & Hewett 1993; Francis et al. 1997; Ellison et al. 2001), although there is currently no evidence that DLAs cluster strongly with LBGs (Gawiser et al. 2001). In addition, the presence of metal line profiles with components separated by many hundreds of km s^-1 provides further evidence that DLAs may have near neighbours (e.g. Pettini et al. 1999; Prochaska & Wolfe 1999). However, due to the difficulty in determining the N(H I) for individual components, it has so far not been possible to perform full abundance analyses of these proximate absorbers.

Here we present high resolution spectroscopic observations of a DLA pair (i.e. two proximate absorbers in the same line of sight) for which the abundances of the individual galaxies can be studied in detail (Sect. 2). In addition to determining column densities for several metal line transitions, the UVES spectra presented here have permitted us to resolve the two Lyman $\alpha$ lines, allowing us to determine values for N(H I) and therefore calculate abundances (Sect. 3). Various explanations for the unusual abundances exhibited by this DLA pair are discussed (Sect. 4), including dust and photo-ionization. Since the chance alignment of two DLAs in single sightline is small, we also consider the possibility that this double absorber is part of some galaxy structure at $z \sim 2$and therefore whether their unusual abundances may be attributed to their environment.