tokens: We thank Jeremiah P. Ostriker, Gurtina Besla, and Benedikt Diemer for useful comments and discussions. SG acknowledges support provided by NASA through Hubble Fellowship grant HST-HF2-51341.001-A awarded by the STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. VS acknowledges support through the European Research Council through ERC-StG grant EXAGAL-308037. LH acknowledges support from NSF grant AST-1312095 and NASA grant NNX12AC67G. Simulations were run on the Harvard Odyssey and CfA/ITC clusters, the Ranger and Stampede supercomputers at the Texas Advanced Computing Center as part of XSEDE, the Kraken supercomputer at Oak Ridge National Laboratory as part of XSEDE, the CURIE supercomputer at CEA/France as part of PRACE project RA0844, and the SuperMUC computer at the Leibniz Computing Centre, Germany, as part of project pr85je.
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