Mass and Redshift Dependence of Star Formation in Relaxed Galaxy Clusters

We investigate the star-formation properties of dynamically relaxed galaxy clusters as a function of cluster mass for 308 low-redshift clusters drawn from the Sloan Digital Sky Survey (SDSS) C4 cluster catalog. It is important to establish if cluster star-formation properties have a mass dependence before comparing clusters at different epochs, and here we use cluster velocity dispersion, σ, as a measure of cluster mass. We select clusters with no significant substructure, a subset of the full C4 sample, so that velocity dispersion is an accurate tracer of cluster mass. We find that the total stellar mass, the number of star-forming galaxies, and total star-formation rate scale linearly with the number of member galaxies, with no residual dependence on cluster velocity dispersion. With the mass-dependence of cluster star-formation rates established, we compare the SDSS clusters with a sample of z ≃ 0.75 clusters from the literature and find that on average (correcting for the mass growth of clusters between the two redshifts) the total Hα luminosity of the high-redshift clusters is 10 times greater than that of the low-redshift clusters. This can be explained by a decline in the Hα luminosities of individual cluster galaxies by a factor of up to ∼ 10 since z ≃ 0.75. The magnitude of this evolution is comparable to that of field galaxies over a similar redshift interval, and thus the effect of the cluster environment on the evolution of star-forming galaxies is at most modest. Our results suggest that the physical mechanism driving the evolution of cluster star-formation rates is independent of cluster mass, at least for clusters with velocity dispersion greater than 450 km s, and operates over a fairly long timescale such that the star-formation rates of individual galaxies decline by an order of magnitude over ∼7 billion years. Subject headings: galaxies: clusters