Environmental effects on Gas and Galaxy Evolution in Clusters

The cluster environment is the site of important transformations in galaxies at a relatively recent cosmological epoch: the galactic morphological types and star formation activity have evolved significantly during the last few Gyrs. How is this related to the galactic gas content? The first part of this paper briefly presents the optical observations of galaxies in distant clusters, while the second part focuses on the link between the H i content and the star formation activity, presenting the results of a multiwavelength study of the cluster A2670 (z=0.08) where 42 galaxies have been detected in H i. We show the star formation properties of the H i-normal and H i-deficient galaxies and discuss their dependence on the location within the cluster. Finally, we compare A2670 with clusters at intermediate redshift and with the Coma cluster. 1. Galaxy evolution in clusters at optical wavelengths Galaxy evolution in clusters has some interesting characteristics: a) it is rapid, because significant evolution is observed already between z ∼ 0.2 − 0.3 and z = 0; b) it mainly involves disk galaxies. As we will discuss, most of the action is taking place in spiral and S0 galaxies while not much happens to the bulk of the ellipticals except passive evolution, at least up to z ∼ 0.6. c) from the optical point of view two types of galaxy evolution are observed: a transformation of the galactic morphologies (i.e. Hubble types) and a change in the star formation activity, therefore the stellar population content. Most of the distant cluster work presented in this section is a result of the MORPHS collaboration (involving W. Couch, A. Dressler, R. Ellis, A. Oemler, B.M. Poggianti and I. Smail) that studied 10 rich clusters at z = 0.4 − 0.5 using WFPC2-HST imaging and ground-based optical spectroscopy (Smail et al. 1997, Dressler et al. 1999 [D99]). 1.1. Morphological evolution The relative frequency of the various Hubble types evolves with redshift. Roughly speaking, spirals are a factor of 2 to 3 more abundant at z ∼ 0.5 than at z = 0, while S0 galaxies are proportionally less abundant in the distant clusters (Dressler et al. 1997 [D97]). Hence, a large number of the S0 galaxies we observe in clusters today seem to have evolved from the numerous spirals observed at