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 24 mu m and GALEX near-ultraviolet 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 2r(200), but remains well below field values even at 3r(200). 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 r(200) 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 similar to 15x from the core to 2r(200). This requires star formation to survive within recently accreted spirals for 2-3 Gyr 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(nu) at 0.3r(500), 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 r(500). All of these results are consistent with star-forming cluster galaxies being an infalling population, but one that must also survive similar to 0.5-2 Gyr beyond passing within r(200). 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 star formation rates decline exponentially on quenching timescales of 1.73 +/- 0.25 Gyr upon accretion into the cluster.