Introduction

Rich clusters of galaxies are fundamental corner-stones of the large-scale structure seen throughout the universe. They are massive (M M), dense systems seen on scales of megaparsecs which appear to be at the intersections of the filaments, chains and sheets which characterise the large-scale structure of galaxies. Therefore they are clearly central to the growth of large-scale structure and thus are ideal laboratories for studying this process. Furthermore, because they contain rich collections of galaxies all at the same distance and within a relatively small region of sky, they provide a very efficient means for studying volume-limited samples of galaxies and investigating the effects the high density environment has on galaxy evolution.

Most of our knowledge of clusters has come from intensive studies of those in the local universe, in particular a small number of very well studied examples such as the Coma and Virgo clusters. Much research has been devoted to understanding the distribution and morphologies of galaxies within these clusters and the overall cluster dynamics (for a recent review see Bahcall 1997). Nearby, regular clusters are dominated by elliptical and S0 (early-type) galaxies (cf. 70% late-type galaxies in the field; Bahcall 1997), and their cores are almost completely devoid of star-forming galaxies. These observations shaped the classical view of clusters being relaxed systems whose galaxies have been inactive in star formation for a large fraction of a Hubble time. However, pioneering work by Butcher and Oemler (1978) showed that clusters at redshifts had significant numbers of blue galaxies in their cores compared to their present day counterparts. This effect - commonly called the ``Butcher-Oemler'' effect - is now known to be a general property of rich, compact clusters at with a clear trend for the blue galaxy fraction to increase with redshift (Couch 1981, Butcher & Oemler, 1984).

The high resolution capabilities of the Hubble Space Telescope (HST) has enabled detailed morphological studies of cluster galaxies at high redshift. Recent results from Couch et al. (1998) and Dressler et al. (1997) show that the blue population consists primarily of late-type spirals, a number of of which are undergoing dynamical interactions and merging. One popular scenario for the origin of this population is dynamical infall, the blue galaxies being field spirals which through their encounter with the intracluster medium and strong gravitational field undergo enhanced star formation before being morphologically transformed into S0s (Abraham et al. 1996, Smail et al. 1997). Evidence has also been found that the end-products of this process may still be seen in the outer regions of nearby clusters with Caldwell et al. (1993) finding galaxies with the same post-starburst spectral signatures of the distant cluster populations in the outskirts of the Coma cluster.

Such low redshift studies have also revealed quite clear evidence of clusters having undergone significant dynamical evolution at recent epochs. This has been through the detection of significant substructure in the velocity and mass distributions within clusters as traced by their galaxies' dynamics (Colless & Dunn 1996), their hot X-ray gas (White et al. 1993) and their gravitational lensing of background sources (Fort & Mellier 1994). A particular striking example of this was published by Zabludoff & Zaritsky (1995; hereafter ZZ95) who demonstrated the power of combining spectroscopic redshifts with X-ray imaging to study the dynamical state of Abell 754. Their extensive study revealed a bi-modal galaxy distribution which was significantly offset from the X-ray emission, consistent with the displacement seen during the collision of two subclusters in hierarchical clustering simulations (Evrard 1990). During such a collision, the models predict that the collisional X-ray gas is stripped from the free-streaming dark matter and galaxies and remains out of hydrostatic equilibrium with the dark matter potential for 1-2Gyr afterwards. Furthermore, the frequency of such subclustering and merging at a particular epoch is sensitive to the mean mass density parameter, (Richstone et al. 1992; Lacey & Cole 1992). A low universe has an early epoch of rapid cluster formation with more regular, spherically symmetric systems at recent times (Evrard et al. 1993), while a high universe has cluster formation (via subcluster merging) occurring at just recent times.

The fact that the epoch of rapid cluster galaxy evolution coincides with one of major cluster dynamical evolution raises the very interesting question: to what extent is the former driven by the latter? To address this question, we are undertaking a major survey of rich clusters in the redshift interval . The aim is to track cluster galaxy evolution and cluster dynamical evolution in tandem over this important transitionary interval in look-back time, thereby bridging the gap between the distant and present-day cluster studies. In addition, it is an interval where a large, homogeneous and representative sample of clusters can be compiled and studied through X-ray selection. By studying a statistically significant number of clusters and thus a representative sample of the cluster population, we shall be able to draw robust conclusions on the overall subcluster merging rate within clusters at this late epoch and thus place useful constraints on .

The outline of this paper is as follows: In §2 we outline the design of our study, its scientific goals and the selection of our cluster sample. The specific observations being undertaken for this project are described in §3. We conclude the paper by presenting some initial results in §4.

© Copyright Astronomical Society of Australia 1997