Galaxy evolution in clusters Clusters of galaxies are important laboratories for the study of the physical processes that drive galaxy evolution. It is likely that the characteristics of the cluster environment (high galaxy density and the presence of a hot diffuse gas component) play a fundamental role in shaping the properties of cluster galaxies, which markedly differ from those of field galaxies. My main research lines in this field are: 1) the determination of the relative distributions of different cluster galaxy populations, and of their orbital characteristics; 2) the detection and characterization of starburst (and post-starburst) galaxies and galaxies with active galactic nuclei (AGN) in clusters, and the way the starburst/AGN phenomenon is related to the properties and dynamical status of the galaxies parent cluster These analyses are partly based on an IR survey of nearby clusters being conducted with the Spitzer Space Telescope, complemented with photometric and spectroscopic data from ground-based telescopes. Fig.1: Galaxies in the cluster Abell 1763 at z=0.23. Green circles indicate the central virialized regions (r500 radius) of the cluster and its neighbor Abell 1770. A multiwavelength survey of the Abell 1763 central region and outskirts has recently been performed, using ground-based optical spectroscopy and photometry (WIYN, Hale, TNG telescopes) and IR photometry from the Spitzer Space Telescope. The star-formation rates of cluster members has been obtained by fitting model templates to their spectral energy distributions (see figure 2). The IR photometry is of course essential to this purpose, because dust obscuration re-directs the energy output of forming stars to longer wavelengths. Many galaxies have a star-formation rate that is very high for their gas content, suggesting they are undergoing a burst of star formation. One galaxy is identified as an AGN from its optical lines. Starburst galaxies are marked with blue stars in the figure, the AGN with a pink triangle. Galaxies with a low (or zero) level of star formation are indicated with empty red dots. The shaded regions highlight two galaxy filaments, probably connecting the two clusters. In these filaments there is a significant excess of starburst galaxies compared to other cluster regions. The burst episode is capable of using up much or all of the gas content of these galaxies, accelerating their evolution to passively evolving red galaxies. Such a discovery suggests that the speeding up of galaxy evolution in clusters occurs in the cluster outskirts, rather than in their central regions, and allows one to identify the process(es) responsible for such an evolution – see Fadda, Biviano et al. 2008. Fig.2: The spectral energy distribution (black dots) of a galaxy in the cluster A1763 (see caption to the figure 1). The blue line shows the best-fit model template, taken from GRASIL (Silva et al. 1998), a young star-forming system -- see Biviano et al. 2004, Fadda, Biviano et al. 2008. Fig.3: The figure displays the relation between the average fraction of optically-identified AGNs in clusters and the velocity dispersions of the clusters they belong to. Blue and green dots are for two different cluster samples, both drawn from the SDSS data-base, one X-ray selected (green dots: the RASS-SDSS sample) and one optically selected (blue dots: the C4 sample). The AGN fraction is seen to increase with decreasing velocity dispersion of the system where the AGNs are located. This trend can be fitted by a galaxy-merging model (Mamon 1992; red line). This suggests a link between the formation of AGNs and galaxy-galaxy mergers which are suppressed in the hottest galaxy systems. The trend inversion for velocity dispersions smaller than 350 km/s suggests however that another additional mechanism is at work -- see Popesso & Biviano 2006. Fig.4: Using the CIRS cluster sample extracted from the SDSS, an average mass profile was determined for the stacked cluster sample through the analysis of the caustics described by cluster galaxies in projected phase-space. Using this mass profile, the Jeans analysis was inverted to determine the orbits of cluster galaxies. The sample was separated in two: red and blue galaxies. The result is shown in the figure. The orbital anisotropy, i.e. the ratio of the radial to tangential velocity dispersions, is shown as a function of cluster radius. Red galaxies have nearly isotropic orbits, and blue galaxies have orbits that become more radial at larger clustercentric distances. Blue galaxies therefore retain memory of their infall from the field, while no such memory is present in the red galaxy population. This is a direct evidence for the accretion process by which clusters grow. It indicates moreover that red galaxies are unlikely to have arrived into the clusters recently, or their orbits would reflect the radial anisotropy typical of an infalling population. Rather, their isotropy suggests they formed together with the cluster, in a violent relaxation process -- see Biviano & Katgert 2004 for asimilar analysis on the ENACS data-set; the analysis of the CIRS sample is currently ongoing.