Reconstructing the history of star formation in rich cluster cores

Our study begins by revisiting the photometric Butcher-Oemler effect using data from seven CNOC clusters at 0.23 less than or similar toz <0.43. We construct the foreground/background-corrected colour-magnitude diagrams for these clusters. Our analysis shows that the CNOC clusters reproduce the trend of increasing blue galaxy fraction with redshift as seen by Butcher & Oemler. We use these data to investigate the history of star formation in clusters by connecting these clusters as snapshots at different redshifts. We address two key issues. First, we ask whether the simple fading and passive evolution of the blue galaxies is consistent with the properties of galaxies in nearby clusters, such as the Coma cluster. We find that the evolution of star-forming field galaxies towards redder colour (once the star formation ceases on entry into the cluster environment) can successfully reconstruct colours and magnitudes of galaxies in the local cluster. There is no requirement for widespread disruption of these galaxies. Since the blue galaxies fade as they age, the fainter galaxies on the present-day colour-magnitude relation tend to have more extended star formation histories than their bright counterparts. However, this effect is not sufficient to cause a sizeable age variation for the galaxies along the colour-magnitude relation, implying that the slope is dominated by variations in metal abundance. Secondly, we address the nature of the Butcher-Oemler effect itself. We compare the distribution of galaxies in the colour-magnitude diagrams and hence infer the evolution of the rate at which galaxies have arrived in the cluster. Models in which star formation is abruptly truncated as galaxies are accreted by the cluster have difficulty in reproducing the observed colour distribution. In contrast, if star formation declines on a 1-Gyr time-scale after accretion, the galaxy accretion history we infer is consistent from cluster to cluster and matches well the distribution expected in simple theoretical models. The Butcher-Oemler effect is thus driven both by the declining star formation rates of field galaxies and by a decline in the rate at which fresh galaxies are accreted by the cluster. Our study naturally leads to a comparison of the global star formation histories of galaxies in clusters and the field. We show that the star formation rate per galaxy mass for galaxies in cluster cores is significantly smaller than that of the field environment below z <1 due to the truncation of star formation. However, the factor by which star formation is suppressed is dependent on the cluster accretion history. High-quality observations of clusters at higher redshifts are needed to define this relation better.