The Role of Mergers in Galaxy Evolution

In the last decade the importance of mergers in the evolution of galaxies has become evident. In this paper we illustrate this importance by showing examples of merging galaxies. both local and at increasing redshift. However before getting carried away by the charms of the hierarchical model in which large galaxies have been built up by successive mergers of smaller objects, it is worth looking at what stellar population synthesis can tell us. Here I show that, using indices which allow us to separate the effects of metallicity and age on the spectra of the stellar populations of galaxies, we can show that the most massive galaxies have the oldest stellar populations, an effect which is enhanced within galaxy Clusters and is maximized within the most massive clusters. These measurements imply that a model where mergers (even "dry" mergers) are the main driver for galaxy evolution cannot be giving us anything like a valid picture. The role of mergers must be considerably more subtle than one would infer front the standard semi-analytic models of galaxy evolution within a cosmological framework.