Forming early-type galaxies in groups prior to cluster assembly

We study a unique protocluster of galaxies, the supergroup SG1120-1202. We quantify the degree to which morphological transformation of cluster galaxies occurs prior to cluster assembly in order to explain the observed early-type fractions in galaxy clusters at z p 0. SG1120-1202 at z-0.37 is composed of four gravitationally bound groups that are expected to coalesce into a single cluster by z p 0. Using HST ACS observations, we compare the morphological fractions of the supergroup galaxies to those found in a range of environments. We find that the morphological fractions of early-type galaxies (∼60%) and the ratio of S0 to elliptical galaxies (0.5) in SG1120-1202 are very similar to clusters at comparable redshift, consistent with preprocessing in the group environment playing the dominant role in establishing the observed early-type fraction in galaxy clusters.