TO APPEAR IN ASTROPHYSICAL JOURNAL Preprint typeset using LATEX style emulateapj v. 12/14/05 INFALLING FAINT [O II] EMITTERS IN ABELL 851. II. ENVIRONMENT, KINEMATICS, & STAR FORMATION HISTORY

We report on the local environment, radial velocity, and star formation history of [O II]-selected emissionline objects in the z ≈ 0.4 galaxy cluster Abell 851, using optical spectra obtained with Keck I/LRIS and Keck II/DEIMOS. A large fraction (≈ 55%) of cluster [O II] emitters show strong Balmer absorption lines (& 4 Å in equivalent width at Hδ). These e(a)-type spectra have been attributed to dusty starburst galaxies by Poggianti & Wu (2000), an interpretation supported by our Balmer-decrement–derived reddening measurements, which show a high frequency of very reddened [E(B−V ) & 0.5] [O II] emitters. Our spectral modeling requires starburst ages . 1 Gyr, which is 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 [O II] emitters, which present a deficit of systems near the cluster systemic velocity compared to a virialized population (or a backsplash population). The spatial segregation of some redshifted and blueshifted groups strongly indicates that the accretion was recent. Throughout the cluster, the fraction in [O II]-emitting galaxies is a strong function of the local galaxy density, being suppressed in dense environments. Our analysis supports previous suggestions that dusty starburst galaxies arise at the expense of continuously star-forming gas-rich spiral galaxies [i.e., the e(c)-type; Dressler et al. (1999)]. In addition, we describe a fainter [O II]-emitting population, comprised largely of dwarf galaxies (Martin et al. 2000) and find an even stronger suppression of [O II] emission in high density environments among this subsample, indicative of more effective destruction by harassment and/or gasstripping. Comparison to previous morphological studies, limited to the central region of Abell 851, suggests that galaxy-galaxy interactions may trigger the starbursts. The high e(a) galaxy fraction in Abell 851 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. Subject headings: galaxy cluster: general — galaxies: clusters: individual(Abell 851;CL0939+4713) — galaxies: dwarf — galaxies: evolution — galaxies: starburst — cosmology: observations