Dynamical effects on galaxies in clusters

The observed morphological evolution since z ∼ 0.5 within galaxy clusters provides evidence for a hierarchical universe. This evolution is driven by dynamical effects that operate within the cluster environment – suppression of star-formation by ram-pressure and viscous stripping of the intra-galactic medium and tidal heating of disks by grav-itational encounters. 1. Evidence for a hierarchical universe? A hierarchical universe with a low matter density is the currently favoured model for structure formation. However, observational evidence for evolution is scarce-structures appear old and similar over moderate look back times (Peebles 1999). Galaxy clusters are the most massive virialised objects in the universe and are the latest systems to form, therefore they may reveal evidence for recent evolution. Indeed, perhaps the strongest indication for evolution can be found by examining galaxy morphologies in rich clusters over the past 5 Gyrs (i.e. At z ∼ 0.5 we see that clusters of galaxies contain predominantly late type disk galaxies that have undergone a transformation into dwarf ellipticals (dSph) by the present day. There is also compelling evidence that the fraction of S0 galaxies decreases over the same look back time (Dressler et al. 1997), whilst at even earlier epochs, tentative evidence for the hierarchical growth of the cluster elliptical galaxies may be found (van Dokkum et al. 1999). The data suggest that the cluster environment is causing a morphological transformation between galaxy types. Other indicators of environmental influences in clusters include the morphology-density relation (Dressler 1980), signatures of star-bursts followed by a rapid truncation of star-formation in cluster spirals and k+a/a+k galaxies (Poggianti et al. 1999), the absence of low surface brightness disks in clusters (Bothun et al. 1993) and the presence of a large component of diffuse starlight (Bernstein et al. 1993, Freeman-this volume). In this paper we will discuss the role of environment and the proposed mechanisms that are responsible for driving galactic evolution in clusters and creating the " morphological and spectroscopic Butcher-Oemler effects " .