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.
