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 counter-parts. We discuss what we can learn about the impact of environment on the global star formation history of the universe.