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 μ 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 2 r 200 , but remains well below field values even at 3 r 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 ∼15 × from the core to 2 r 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 σ ν at 0.3 r 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 ∼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.