tokens: The authors thank Steve Finkelstein for helpful discussions, and the anonymous referee for insightful comments on the manuscript. AGS was supported by an AGEP-GRS supplement to NSF grant AST-1009973. MBK acknowledges support provided by NASA through Hubble Space Telescope theory grants (programs AR-12836 and AR-13888) from the Space Telescope Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555. DRW is supported by NASA through Hubble Fellowship grant HST-HF-51331.01 awarded by the Space Telescope Science Institute. JSB was supported by HST AR-12836 and NSF AST-1009973.
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