The Difference Between Clusters and Groups : A Journey from Cluster Cores to their Outskirts and Beyond .

In this review, we take the reader on a journey. We start by looking at the properties of galaxies in the cores of rich clusters. We have focused on the overall picture: star formation in clusters is strongly suppressed relative to field galaxies at the same redshift. We will argue that the increasing activity and blue populations of clusters with redshift results from a greater level of activity in field galaxies rather than a change in the transformation imposed by the cluster environment. With this in mind, we travel out from the cluster focussing firstly on the properties of galaxies in the outskirts of clusters and then on galaxies in isolated groups. At low redshift, we are able to efficiently probe these environments using the Sloan and 2DF redshift surveys. These allow an accurate comparison of galaxy star formation rates in different regions. The current results show a strong suppression of star formation above a critical threshold in local density. The threshold seems similar regardless of the overall mass of the system. At low redshift at least, only galaxies in close, isolated pairs have their star formation rate boosted above the global average. At higher redshift, work on constructing homogeneous catalogues of galaxies in groups and in the infall regions of clusters is still at an early stage. In the final section, we draw these strands together, summarising what we can deduce about the mechanisms that transform star forming field galaxies into their quiescent cluster counterparts. We discuss what we can learn about the impact of environment on the global star formation history of the universe. 1.1 Introduction Let me start with an outline of this review. We will begin by looking at galaxies in the cores of clusters. We have been observing clusters for many years. Some milestones are the papers on the morphological differences between cluster galaxies and the general field (Hubble & Humason, 1931), the discovery of a global morphology-density relation (Oemler 1974, Dressler 1980), and the realisation of the importance of the colour-magnitude relation (Sandage & Visvanathan, 1978). We will attempt to summarise what we have learned from looking at clusters since this time. In particular, recent observations now span a wide range of redshift, allowing us to look directly at how the galaxy populations evolve. In the second section, we will investigate how galaxy star formation rates vary with radius and local density. In particular, …