Galaxy populations in the field and in clusters are markedly different, and yet clusters keep forming and growing by the continuous infall of field galaxies. The field galaxy population is rich in spirals like our Milky Way with active star formation (=SF), while the inner parts of rich galaxy clusters are dominated by S0 and dwarf galaxies without any gas and SF. A variety of processes have been proposed, and indeed observed to work in individual cases, that can transform gas-rich SFing field spirals into gas-poor passive cluster S0s or dwarfs. Some processes are due to interactions of the infalling spirals with the hot and dense intracluster gas observed in X-rays in the central regions of rich clusters (ram pressure stripping/sweeping), others have to do with the frequent high-speed encounters between galaxies (harassment), or with the increased galaxy merger rate within infalling groups. All processes occur preferentially at different distances from the cluster center, probably have different timescales and transition stages (blue/red E+A/Hdelta-strong galaxies), and, partly, different end products (S0s, dEs, dSphs). What are the dominant process(es), their respective transition stages, and timescales, and whether (and how) these depend on the cluster properties, are all still open questions, despite considerable effort in the last decade.

In a collaborative project with R. Kraan-Korteweg and P. Woudt from the University of Cape Town (UCT), we attempt for the first time to reach a complete census of the relative importance of the various transformation channels in  clusters of various richness and degree of virialization (=regularity in shape), and to track in detail on a large statistical basis the transition paths (types and timescales) for the various transformation processes. Data are available to us from archives, from our collaboration with the MORPHs group (A. Dressler, Pasadena, B. Poggianti, Padova), taken by the UCT group at various telescopes, and, ultimately, with the unprecedentedly deep and wide-field multi-band imaging of galaxy clusters with the South African Large Telescope (SALT), in combination our GALEV  evolutionary synthesis models of galaxies of various types that can account for the various transformation processes in their impact on the SF history and, consequently, on the spectral evolution of the infalling galaxies. The key point, in which our approach will be superior to what was done before, is that it will, in a first step, use deep multi-band imaging, which gives access to much larger numbers of galaxies and to the dominant population of fainter ones than spectroscopy. The unique U-band (=short wavelength) sensitivity of SALT in combination with optical and near-infrared imaging allows, as our modelling has shown, to derive photometric redshifts (=distances) accurate enough for membership identification and to discriminate between the different scenarios in different stages (Fritze et al. 2006, IAU Symp. 232, 508). Stellar masses and metallicities of all the galaxies come as a by-product. Only in a small fraction  of ambiguous cases will additional spectroscopy be required in a second step, then allowing to compare stellar to total masses and assess the DM content. Preparatory work for this project has been done by J. Bicker and M. A. Tyra in their Diploma theses in Göttingen (Bicker et al. 2002, Fritze & Bicker 2006, Tyra 2007 (Dipl.-arb. Uni Göttingen), Tyra & Fritze in prep.. The first PhD thesis at UCT on this subject is underway in close collaboration with our group. P. Kotze has obtained and reduced NIR JHK imaging for Abell 1437 which has high-quality SDSS data (photometry and redshifts). He is currently visiting at Univ. of Hertfordshire to start the analysis with GALEV models.

Clusters at different redshifts and of various degrees of richness and relaxation are to be analysed this way out to a few virial radii before a full census of the transformation processes, timescales, transition stages, and end products and their respective dependences on cluster properties will be achieved. Comparison of spectral transformation timescales with morphological information from HST images will allow to compare to timescales for the morphological transformation and, ultimately, to understand the interplay between Large Scale Structure formation and galaxy formation/evolution.

References:
- Bicker et al, A&A 387, 412 (2002)
- Fritze & Bicker, A&A 454, 67 (2006)
- Fritze & Woudt, IAUS 232, 223 (2006)
- Kotze et al, in prep
- Falkenberg & Fritze, 2008 (accepted to MNRAS)
- Falkenberg, Kotulla & Fritze, 2009 (submitted to MNRAS)