LoCuSS: THE STEADY DECLINE AND SLOW QUENCHING OF STAR FORMATION IN CLUSTER GALAXIES OVER THE LAST FOUR BILLION YEARS

We present an analysis of the levels and evolution of star formation activity in a representative sample of 30 massive galaxy clusters at 0.15 < z < 0.30 from the Local Cluster Substructure Survey, combining wide-field Spitzer/MIPS 24 mu m data with extensive spectroscopy of cluster members. The specific SFRs of massive (M greater than or similar to 10(10) M-circle dot) star-forming cluster galaxies within r(200) are found to be systematically similar to 28% lower than their counterparts in the field at fixed stellar mass and redshift, a difference significant at the 8.7 sigma level. This is the unambiguous signature of star formation in most (and possibly all) massive star-forming galaxies being slowly quenched upon accretion into massive clusters, their star formation rates (SFRs) declining exponentially on quenching timescales in the range 0.7-2.0 Gyr. We measure the mid-infrared Butcher-Oemler effect over the redshift range 0.0-0.4, finding rapid evolution in the fraction (f(SF)) of massive (M-K < -23.1) cluster galaxies within r(200) with SFRs > 3 M-circle dot yr(-1), of the form f(SF) proportional to(1 + z)(7.6 +/- 1.1). We dissect the origins of the Butcher-Oemler effect, revealing it to be due to the combination of a similar to 3x decline in the mean specific SFRs of star-forming cluster galaxies since z similar to 0.3 with a similar to 1.5x decrease in number density. Two-thirds of this reduction in the specific SFRs of star-forming cluster galaxies is due to the steady cosmic decline in the specific SFRs among those field galaxies accreted into the clusters. The remaining one-third reflects an accelerated decline in the star formation activity of galaxies within clusters. The slow quenching of star formation in cluster galaxies is consistent with a gradual shut down of star formation in infalling spiral galaxies as they interact with the intracluster medium via ram-pressure stripping or starvation mechanisms. The observed sharp decline in star formation activity among cluster galaxies since z similar to 0.4 likely reflects the increased susceptibility of low-redshift spiral galaxies to gas removal mechanisms as their gas surface densities decrease with time. We find no evidence for the build-up of cluster S0 bulges via major nuclear starburst episodes.