The Galaxies.

Clusters of galaxies were originally identified as regions of overdensities of galaxies on photographic plates in the pioneering work of Abell (1958) and Zwicky et al. (1961-1968)). The number and distribution of the galaxies in clusters have been used to construct several different schemes for classifying cluster types. For example, Abell (1958) classified clusters into richness classes according to the number of galaxies in a cluster which are brighter than $m_{3}+2$ where $m_{3}$ is the magnitude of the third brightest cluster member. Broadly speaking, clusters vary from rich clusters containing 1000s of galaxies and a centrally concentrated, symmetrical distribution through to poor clusters, or groups, containing 10s of galaxies and an irregular distribution.

Galaxies in clusters are predominantly elliptical (E) and lenticular (S0) early-types. The morphological mix of galaxies was shown by Dressler (1980) to be strongly dependent on the local projected density, with larger densities showing an increased fraction of Es and S0s and less dense regions having a larger fraction of spirals (Sps). Whitmore & Gilmore (1991) and Whitmore et al. (1993) have shown that this relation holds true when reinterpreted in terms of `cluster-centric' radius, with the fraction of early-types increasing towards the cluster core. A further variation in morphological type, shown by Driver et al. (1998) and Andreon (2001), is the increase of dwarf-to-giant galaxy ratio towards the outer regions of clusters.

Variation in galaxy properties, such as number density, luminosity, colour and morphological-mix, are excellent tools with which to test theories of galaxy formation. The evolution of galaxies in clusters is the main theme of chapters 4, 5 and 6.