LoCuSS: The mid-infrared Butcher-Oemler effect

We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02<z<0.40, using panoramic Spitzer/MIPS 24micron and NIR data. This is the largest sample of clusters to date with MIR data covering not only the cluster cores, but extending into the infall regions. We revisit the Butcher-Oemler effect, measuring the fraction of massive infrared-luminous galaxies (K<K*+1.5, L_IR>5x10^10L_sun) within r_200, finding a steady increase in the fraction with redshift from ~3% at z=0.02 to ~10% by z=0.30, and an rms cluster-to-cluster scatter about this trend of 0.03. The best-fit redshift evolution model is of the form f_SF ~ (1+z)^5.7, which is stronger redshift evolution than that of L*_IR in both clusters and the field. We find that, statistically, this excess is associated with galaxies found at large cluster-centric radii, implying that the MIR Butcher-Oemler effect can be explained by a combination of both the global decline in star-formation in the universe since z~1 and enhanced star formation in the infall regions of clusters at intermediate redshifts. This picture is supported by a simple infall model based on the Millennium Simulation semi-analytic galaxy catalogs, whereby star-formation in infalling galaxies is instantaneously quenched upon their first passage through the cluster, in that the observed radial trends of f_SF trace those inferred from the simulations. We also find that f_SF does not depend on simple indicators of the dynamical state of clusters, including the offset between the brightest cluster galaxy and the peak of the X-ray emission. This is consistent with the picture described above in that most new star-formation in clusters occurs in the infall regions, and is thus not sensitive to the details of cluster-cluster mergers in the core regions.