Infalling faint [O II] emitters in Abell 851. I. Spectroscopic confirmation of narrowband-selected objects

We report on a spectroscopic confirmation of narrowband-selected [O II] emitters in Abell 851 cataloged by Martin and coworkers. The optical spectra obtained from the Keck I Low Resolution Imaging Spectrometer (LRIS) and Keck II Deep Imaging Multi-Object Spectrograph (DEIMOS) have confirmed [O II] lambda 3727 emission in narrowband-selected cluster [O II] candidates at a approximate to 85% success rate for faint (i less than or similar to 25) blue (g - i < 1) galaxies. The rate for the successful detection of [O II] emission is a strong function of galaxy color, generally proving the efficacy of narrowband [O II] search supplemented with broadband colors in selecting faint cluster galaxies with recent star formation. Balmer decrement-derived reddening measurements show a high degree of reddening [E(B - V) greater than or similar to 0.5] in a significant fraction of this population. Even after correcting for dust extinction, the [O II]/H alpha line flux ratio for the high-E(B - V) galaxies remains generally lower by a factor of similar to 2 than the mean [O II]/H alpha ratios reported by the studies of nearby galaxies. The strength of [O II] equivalent width shows a negative trend with galaxy luminosity, while the H alpha equivalent width does not appear to depend as strongly on luminosity. This in part is due to the high amount of reddening observed in luminous galaxies. Furthermore, emission-line ratio diagnostics show that AGN-like galaxies are abundant in the high-luminosity end of the cluster [O II]- emitting sample, with only moderately strong [O II] equivalent widths, consistent with a scenario of galaxy evolution connecting AGNs and suppression of star-forming activity in massive galaxies.