  We report on the environments, kinematics, and SFH of [OII]-selected objects
in the z=0.4 galaxy cluster A851, using Keck optical spectra. A large fraction
(~55%) of cluster [OII] emitters show strong Balmer absorptions (>4A in
equivalent width at Hd). These e(a) spectra have been attributed to dusty
starburst galaxies, an interpretation supported by our reddening measurements,
which show a high frequency of very reddened [EBV>0.5] [OII] emitters. Our
spectral modeling requires starburst ages >1Gyr, shorter than the cluster
crossing timescale. We argue that this starburst phase occurs during cluster
infall based on the radial velocity distribution of the cluster [OII] emitters,
which present a deficit of systems near the cluster systemic velocity. The
spatial segregation of some redshifted and blueshifted groups strongly
indicates that the accretion was recent. Throughout the cluster, the fraction
in [OII] emitters is a strong function of the local galaxy density. Our
analysis supports previous suggestions that dusty starburst galaxies arise at
the expense of gas-rich spiral galaxies. In addition, we describe a fainter
[OII]-emitting population, comprised largely of dwarf galaxies and find an even
stronger suppression of [OII] emission in high density environments among this
subsample, indicative of more effective destruction by harassment and/or
gas-stripping. Comparison to previous morphological studies, limited to the
core of A851, suggests that galaxy-galaxy interactions may trigger the
starbursts. The high e(a) galaxy fraction in A851 compared to that in the
field, however, suggests that some cluster-specific mechanism, likely related
to the dynamical assembly of the cluster, also contributes to the high number
of starbursts. (abridged)
