Galaxy evolution in poor clusters

Abstract

We study the galaxy population in poor clusters and compare it with the existing results for rich clusters, in an attempt to understand the role of the environment in the formation and evolution of galaxies. Studies of rich clusters of galaxies have revealed dramatic transformations between the population of local and distant clusters. Specifically, distant rich clusters have a higher fraction of blue galaxies and significantly less SO galaxies than their local counterparts. The effectiveness of the candidate mechanisms responsible for these transformations depends on the density of the environment. Our aim is to try to distinguish between these mechanisms. This thesis is part of a larger project comprising of a sample of nine X-ray selected poor clusters in the redshift range 0.2-0.3. These data comprise of ground based photometry, multi-slit spectroscopy and HST images. This work concentrates on four of the nine clusters of the sample. We have obtained Colour-Magnitude Diagrams for these clusters and calculated their blue galaxy fraction. We find values similar to those found for rich clusters at similar redshifts. We also show results from the morphological analysis of the HST images, performed as a side project to this work. The morphological analysis reveals that our clusters have a higher fraction of low B/T systems than rich clusters. We discuss the different candidate mechanisms and argue that the so-called "strangulation" is the only one compatible with our findings. In this scenario, galaxies loose their gas envelope as they are accreted to the cluster and star formation is gradually truncated as the galaxy consumes the rest of its gas. This process does not have a significant effect on the morphology of the infalling galaxy. In rich clusters, where other mechanisms (tides, harassment, ram-pressure stripping) are effective, the morphology of the galaxy will be transformed.