Star Formation in Cluster Galaxies at 0.2 < Z < 0.55

The rest-frame equivalent width of the [OII] λ3727 emission line, W₀(O II), has been measured for cluster and field galaxies in the Canadian Network for Observational Cosmology redshift survey of rich clusters at 0.2<z<0.55. Emission lines of any strength in cluster galaxies at all distances from the cluster center, out to 2R₂₀₀, are less common than in field galaxies. The mean W₀(O II) in cluster galaxies more luminous than M^k_r< -18.5 + 5 log h (q₀=0.1) is 3.8+/-0.3 Å (where the uncertainty is the 1 σ error in the mean), which is significantly less than the field galaxy mean of 11.2+/-0.3 Å. For the innermost cluster members (R<0.3R₂₀₀), the mean W₀(O II) is only 0.3+/-0.4 Å. Thus, it appears that neither the infall process nor internal tides in the cluster induce detectable excess star formation in cluster galaxies relative to the field. The color-radius relation of the sample is unable to fully account for the lack of cluster galaxies with W₀(O II) > 10 Å, as expected in a model of cluster formation in which star formation is truncated on infall. Evidence of suppressed star formation relative to the field is present in the whole cluster sample, out to 2R₂₀₀, so the mechanism responsible for the differential evolution must be acting at a large distance from the cluster center and not just in the core. The mean star formation rate in the cluster galaxies with the strongest emission corresponds to an increase in the total stellar mass of less than about 4% if the star formation is due to a secondary burst lasting 0.1 Gyr.