Galaxy Evolution in Groups and Clusters

Abstract

In this thesis, we investigate the extent to which galaxy evolution is driven by processes common to the group and cluster environments. A bimodality of galaxy properties such as star formation, strongly dependent upon the local overdensity of galaxies, suggests that the passive, early type galaxies common to groups and clusters originate in transformation processes, which are nurtured by the environment. This can only be important to global galaxy evolution if transformations are common in groups, which contain z 50% of the local galaxy population. We present deep Magellan spectroscopy and HST ACS imaging of our group and field samples at 0.3 ˂ z ˂ 0.55, selected from the CN0C2 survey by Carlberg et al. (2001b). We find that these groups contain significantly more passive galaxies than the field, with excesses of S0, elliptical and passive spiral galaxy types. The morphological composition is closely matched to that of irregular and X-ray-faint clusters at a similar epoch. In contrast with a low-redshift group sample selected from 2dFGRS (Eke et al, 2004), we find that the fraction of passive galaxies, fp， is strongly evolving in the group environment, with parallel evolution in the field population. Simple models confirm that galaxy transformations are required to match the evolution of both group and field populations. Qualitatively similar evolution and dependence on environment is found in physically-motivated simulations. However, these do not quantitatively match the environmental nor luminosity dependence in the evolution of fp. We also present a complementary method using photometric redshifts to identify infalling groups in the outskirts of clusters with Wide Field Imaging (WFI) technology. Finally, we identify the key developments which will help to unravel the history of galaxy evolution in coming years.