M ay 2 00 0 An Emission - Line Search for Star - Forming Dwarf Galaxies Toward Abell 851

We present deep images of the redshift z = 0.41 galaxy cluster Abell 851 (Cl 0939+4713) including a narrow bandwidth image of the redshifted [OII] λλ3726, 29 line emission. The [OII] doublet is the most accessible tracer of star formation out to redshifts near unity, and we use it to identify candidates for star-forming galaxies. The galaxies selected lie within a projected clustercentric distance of 2 Mpc and have line-of-sight velocities with respect to the cluster in the range between ± 1460 km s . The sample is complete to magnitude m5129(AB) = 24.0 for galaxies with emission-line equivalent widths greater than about 11 Å in the observed frame. The sample is contaminated by foreground galaxies with a steep 4000 Å break across the filters and possibly Lyα emitters at z=3.31, but such galaxies can be identified using color, brightness, and equivalent-width criteria. Excluding the likely interlopers, the number density of star-forming galaxies near Cl 0939+4713 is still at least three to four times higher than that of a similar column through the field at z=0.4. Galaxies in the cluster sample have lower [OII] equivalent widths than similarly selected field galaxies at z ∼ 0.4 however, and we suggest that while the density of galaxies is higher in the cluster the star-formation activity in individual galaxies is supressed relative to their counterparts in the field. The relatively small enhancement in the surface density of the [OII]-selected population toward the cluster core leads us to believe the [OII]-selected galaxies reside in the outer regions of the cluster. We identify a population of starburst galaxies among the [OII]-emitters based on their blue g-i color. The majority of the [OII]-selected galaxies appear to be related to the normal spiral galaxy population, but the bursting population is dominated by intrinsically faint galaxies of below average size. We suggest that this population is composed primarily of dwarf galaxies