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tokens: Acknowledgments We thank C.R. Christensen, R. Davé, S. Genel, Z. Hafen, A.V. Kravtsov, M. Kriek, A.L. Muratov, B. D. Oppenheimer, A. J. Richings, J. Rojas-Sandoval, R. S. Somerville and S. Veilleux for useful discussions. We greatly appreciate the referee’s thoughtful comments that helped us improve the paper in several places. DAA acknowledges support by a CIERA Postdoctoral Fellowship. CAFG was supported by NSF through grants AST-1412836 and AST-1517491, by NASA through grant NNX15AB22G and by STScI through grants HST-AR-14293.001-A and HST-GO-14268.022-A. DK was supported by NSF grant AST-1412153 and a Cottrell Scholar Award. Support for PFH was provided by an Alfred P. Sloan Research Fellowship, NASA ATP Grant NNX14AH35G, and NSF Collaborative Research Grant #1411920 and CAREER grant #1455342. EQ was supported by NASA ATP grant 12-ATP12-0183, a Simons Investigator award from the Simons Foundation, and the David and Lucile Packard Foundation. The simulations analysed in this paper were run using the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF grant ACI-1053575 (allocations TG-AST120025, TG-AST130039 and TG-AST140023). This work greatly benefited from the hospitality of the Aspen Center for Physics, supported by NSF grant PHY-1066293.