2015ApJ...806..101H

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 µm 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 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 Å15x from the core to 2r₂₀₀. 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 σ_ν 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 Å0.5-2 Gyr 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 star formation rates decline exponentially on quenching timescales of 1.73±0.25 Gyr upon accretion into the cluster.