LoCuSS: THE SLOW QUENCHING OF STAR FORMATION IN CLUSTER GALAXIES AND THE NEED FOR PRE-PROCESSING
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Haines, C. P.
Pereira, M. J.
Smith, G. P.
McGee, S. L.
Rawle, T. D.
Moran, S. M.
galaxies: active – galaxies: clusters: general – galaxies: evolution – galaxies: stellar content
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 (fSF) of star-forming cluster galaxies rises steadily with clustercentric 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 fSF-radius trend, the surface density of star-forming galaxies actually declines steadily with radius, falling ∼15× 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 σ_ν 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 ∼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.
C.P.H. was funded by CONICYT Anillo project ACT-1122. G.P.S. acknowledges support from the Royal Society. F.Z. and G.P.S. acknowledge support from the Science and Technology Facilities Council. We acknowledge NASA funding for this project under the Spitzer program GO:40872. This work was supported in part by the National Science Foundation under grant No. AST-1211349. The Millennium simulation databases used in this paper and the web application providing online access to them were constructed as part of the activities of the German Astrophysical Virtual Observatory.
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