LoCuSS: The slow quenching of star formation in cluster galaxies and the
  need for pre-processing
  We present a study of the spatial distribution and kinematics of star-forming
galaxies in 30 massive clusters at 0.15<z<0.30, combining wide-field Spitzer
24um and GALEX NUV imaging with highly-complete spectroscopy of cluster
members. The fraction (f_SF) of star-forming cluster galaxies rises steadily
with cluster-centric radius, increasing fivefold by 2r200, but remains well
below field values even at 3r200. This suppression of star formation at large
radii cannot be reproduced by models in which star formation is quenched in
infalling field galaxies only once they pass within r200 of the cluster, but is
consistent with some of them being first pre-processed within galaxy groups.
Despite the increasing f_SF-radius trend, the surface density of star-forming
galaxies actually declines steadily with radius, falling ~15x from the core to
2r200. This requires star-formation to survive within recently accreted spirals
for 2--3Gyr to build up the apparent over-density of star-forming galaxies
within clusters. The velocity dispersion profile of the star-forming galaxy
population shows a sharp peak of 1.44-sigma_v at 0.3r500, and is 10--35% higher
than that of the inactive cluster members at all cluster-centric radii, while
their velocity distribution shows a flat, top-hat profile within r500. All of
these results are consistent with star-forming cluster galaxies being an
infalling population, but one that must also survive ~0.5--2Gyr beyond passing
within r200. By comparing the observed distribution of star-forming galaxies in
the stacked caustic diagram with predictions from the Millennium simulation, we
obtain a best-fit model in which SFRs decline exponentially on quenching
time-scales of 1.73\pm0.25 Gyr upon accretion into the cluster.
