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 (LoCuSS), combining wide-field Spitzer
24um data with extensive spectroscopy of cluster members. The specific-SFRs of
massive (M>10^10 M_sun) star-forming cluster galaxies within r200 are found to
be systematically 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 SFRs declining exponentially on quenching time-scales 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 r200 with SFRs>3M_sun/yr, of the
form f_SF (1+z)^7.6. We dissect the origins of the Butcher-Oemler effect,
revealing it to be due to the combination of a ~3x decline in the mean
specific-SFRs of star-forming cluster galaxies since z~0.3 with a ~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 intra-cluster medium via
ram-pressure stripping or starvation mechanisms. We find no evidence for the
build-up of cluster S0 bulges via major nuclear star-burst episodes.
