The IMACS Cluster Building Survey: II. Spectral Evolution of Galaxies in
  the Epoch of Cluster Assembly
  The IMACS Cluster Building Survey (ICBS) provides spectra of ~2200 galaxies
0.31<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 of
the 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) or
starbursts, (SBH or SBO), identified by anomalously strong H-delta 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
starforming 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 tau < 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 starforming 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.
