Infalling Faint [O II] Emitters in Abell 851. II. Environment, Kinematics, and Star Formation History

We report on the local environment, kinematics, and star formation history of [O II]-selected objects in the z ≈ 0.4 galaxy cluster Abell 851, using Keck optical spectra. A large fraction (≈55%) of cluster [O II] emitters show strong Balmer absorptions (≳4 Å in Hδ equivalent width). These e(a)-type spectra have been attributed to dusty starburst galaxies by Poggianti & Wu, an interpretation supported by our reddening measurements, which show a high frequency of very reddened [E(B - V) ≳ 0.5] galaxies. Our spectral modeling requires starburst ages of ≲1 Gyr, which is shorter than the cluster crossing timescale. We argue that this starburst phase occurs during cluster infall on the basis of the radial velocity distribution of the [O II] emitters, which present a deficit of systems near the cluster systemic velocity when 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 presence of [O II] emitters is strongly suppressed in dense environments. Our analysis supports previous suggestions that dusty starburst galaxies arise at the expense of gas-rich spiral galaxies [i.e., the e(c) type; Dressler and coworkers]. In addition, we describe a fainter population comprised largely of dwarf galaxies (Martin and coworkers) and find an even stronger suppression of [O II] emitters in high-density environments, indicative of more effective destruction by harassment and/or gas stripping. Comparison to previous morphological studies, limited to the cluster core, suggests that galaxy-galaxy interactions may trigger the starbursts. The high e(a) galaxy fraction in Abell 851 as 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.