A wide-field survey of two z ∼ 0.5 galaxy clusters:: Identifying the physical processes responsible for the observed transformation of spirals into S0s

We present new results from our comparative survey of two massive, intermediate-redshift galaxy clusters, Cl 0024+17 (z = 0.39) and MS 0451-03 (z = 0.54). Combining optical and UV imaging with spectroscopy of member galaxies, we identify and study several key classes of "transition objects'' whose stellar populations or dynamical states indicate a recent change in morphology and star formation rate. For the first time, we have been able to conclusively identify spiral galaxies in the process of transforming into S0 galaxies. This has been accomplished by locating both spirals whose star formation is being quenched and their eventual successors, the recently created S0s. Differences between the two clusters in both the timescales and spatial location of this conversion process allow us to evaluate the relative importance of several proposed physical mechanisms that could be responsible for the transformation. Combined with other diagnostics that are sensitive to either ICM-driven galaxy evolution or galaxy-galaxy interactions, we describe a self-consistent picture of galaxy evolution in clusters. We find that spiral galaxies within infalling groups have already begun a slow process of conversion into S0s, likely via gentle galaxy-galaxy interactions. The fates of spirals upon reaching the core of the cluster depend heavily on the cluster ICM, with rapid conversion of all remaining spirals into S0s via ram pressure stripping in clusters where the ICM is dense. In the presence of a less dense ICM, the conversion continues at a slower pace, with other mechanisms continuing to play a role. We conclude that the buildup of the local S0 population through the transformation of spiral galaxies is a heterogeneous process that nevertheless proceeds robustly across a variety of different environments.