The Star-forming Dwarf Galaxy Populations of Two Z ∼ 0.4 Clusters: Ms1512.4+3647 and Abell 851

We present the results of a deep narrow-band [OII] 3727 Å emission-line search for faint (g < 27), star-forming galaxies in the field of the z = 0.37 MS1512.4+3647 cluster. We find no evidence for an over-density of emission-line sources relative to the field at z ∼ 0.4 (Hogg et al. 1998), and therefore conclude that the MS1512.4+3647 sample is dominated by field [OII] emission-line galaxies which lie along the ∼ 180 Mpc line of sight immediately in front and behind the cluster. This is surprising, given that the previously surveyed z = 0.41 cluster Abell 851 has 3-4 times the field emission-line galaxy density (Martin et al. 2000). We find that the MS1512.4+3647 sample is deficient in galaxies with intermediate colors (1.0 < g− i < 2.0) and implied star-formation exponential decay timescales τ ∼ 100 Myr 1 Gyr that dominate the Abell 851 emission-line galaxy population. Instead, the majority of [OII] emission-line galaxies surrounding the MS1512.4+3647 cluster are blue (g − i ≤ 1.0) and forming stars in bursts with τ < 100 Myr. In both samples, galaxies with the shortest star-formation timescales are preferentially among the faintest star-forming objects. Their i luminosities are consistent with young stellar populations ∼ 10 − 10M⊙, although an additional factor of ten in stellar mass could be hiding in underlying old stellar populations. We discuss the implications for the star-formation histories of dwarf galaxies in the field and rich clusters.