tokens: We are indebted to the staff of the W.M. Keck Observatory who keep the instruments and telescopes running effectively. We also wish to extend thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. This work has been supported in part by the US National Science Foundation through grants AST-0908805 and AST-1313472. We thank Mariska Kriek and Eliot Quataert for many useful discussions as well as the anonymous referee for comments and suggestions that significantly improved this work. R.F.T. also acknowledges support from Dennis and Carol Troesh and from the Miller Institute for Basic Research in Science at the University of California, Berkeley.
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