2013ApJ...770...62D

The IMACS Cluster Building Survey (ICBS) provides spectra of ∼2200 galaxies 0.31 < z < 0.54 in five rich clusters (R ≲ 5 Mpc) and the field. Infalling, dynamically cold groups with tens of members account for approximately half of the supercluster population, contributing to a growth in cluster mass of ∼100% by the present day. The ICBS spectra distinguish non-star-forming (PAS) and poststarburst (PSB) from star-forming galaxies–continuously star-forming (CSF) or starbursts (SBH or SBO), identified by anomalously strong Hδ absorption or [O II] emission. For the infalling cluster groups and similar field groups, we find a correlation between PAS+PSB fraction and group mass, indicating substantial "preprocessing" through quenching mechanisms that can turn star-forming galaxies into passive galaxies without the unique environment of rich clusters. SBH + SBO starburst galaxies are common, and they maintain an approximately constant ratio (SBH+SBO)/CSF ~ 25% in all environments–from field, to groups, to rich clusters. Similarly, while PSB galaxies strongly favor denser environments, PSB/PAS ~ 10%-20% for all environments. This result, and their timescale τ ∼ 500 Myr, indicates that starbursts are not signatures of a quenching mechanism that produces the majority of passive galaxies. We suggest instead that starbursts and poststarbursts signal minor mergers and accretions, in star-forming and passive galaxies, respectively, and that the principal mechanisms for producing passive systems are (1) early major mergers, for elliptical galaxies, and (2) later, less violent processes–such as starvation and tidal stripping, for S0 galaxies.