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The IMACS Cluster Building Survey (ICBS) provides spectra of ~2200 galaxies0.31<z<0.54 in 5 rich clusters (R <= 5 Mpc) and the field. Infalling,dynamically cold groups with tens of members account for approximately half ofthe supercluster population, contributing to a growth in cluster mass of ~100%by today. The ICBS spectra distinguish non-starforming (PAS) and poststarburst(PSB) from starforming galaxies -- continuously starforming (CSF) orstarbursts, (SBH or SBO), identified by anomalously strong H-delta absorptionor [O II] emission. For the infalling cluster groups and similar field groups,we find a correlation between PAS+PSB fraction and group mass, indicatingsubstantial "preprocessing" through quenching mechanisms that can turnstarforming galaxies into passive galaxies without the unique environment ofrich clusters. SBH + SBO starburst galaxies are common, and they maintain anapproximately constant ratio (SBH+SBO)/CSF ~ 25% in all environments -- fromfield, to groups, to rich clusters. Similarly, while PSB galaxies stronglyfavor denser environments, PSB/PAS ~ 10-20% for all environments. This result,and their timescale tau < 500 Myr, indicates that starbursts are not signaturesof a quenching mechanism that produces the majority of passive galaxies. Wesuggest instead that starbursts and poststarbursts signal minor mergers andaccretions, in starforming and passive galaxies, respectively, and that theprincipal mechanisms for producing passive systems are (1) early major mergers,for elliptical galaxies, and (2) later, less violent processes -- such asstarvation and tidal stripping, for S0 galaxies.

The IMACS Cluster Building Survey: II. Spectral Evolution of Galaxies in the Epoch of Cluster Assembly