The Evolution of [OII] Emission from Cluster Galaxies

We investigate the evolution of the star formation rate in cluster galaxies. We complement data from the CNOC1 cluster survey (0.15 < z < 0.6) with measurements from galaxy clusters in the 2dF galaxy redshift survey (0.05 < z < 0.1) and measurements from recently published work on higher redshift clusters, up to almost z = 1. We focus our attention on galaxies in the cluster core, ie. galaxies with r < 0.7h 70 Mpc. Averaging over clusters in redshift bins, we find that the fraction of galaxies with strong [OII] emission is . 20% in cluster cores, and the fraction evolves little with redshift. In contrast, field galaxies from the survey show a very strong increase over the same redshift range. It thus appears that the environment in the cores of rich clusters is hostile to star formation at all the redshifts studied. We compare this result with the evolution of the colours of galaxies in cluster cores, first reported by Butcher & Oemler (1984). Using the same galaxies for our analysis of the [OII] emission, we confirm that the fraction of blue galaxies, which are defined as galaxies 0.2 mag bluer in the rest frame B − V than the red sequence of each cluster, increases strongly with redshift. Since the colours of galaxies retain a memory of their recent star formation history, while emission from the [OII] line does not, we suggest that these two results can best be reconciled if the rate at which the clusters are being assembled is higher in the past, and the galaxies from which it is being assembled are typically bluer.