tokens: The authors thank the anonymous reviewer for a careful reading of this manuscript and providing thoughtful feedback that improved this paper. We also thank Alis Deason for assistance in line-list vetting, Gina Duggan for useful discussions on generating grids of synthetic spectra, Raja Guha Thakurta for help with observations and insightful conversations, and Luis Vargas and Marla Geha for sharing their data for M31 outer halo RGB stars. I.E. acknowledges support from a Ford Foundation Predoctoral Fellowship and the NSF Graduate Research Fellowship under grant No. DGE-1745301, as well as the NSF under grant No. AST-1614081, along with E.N.K. K.M.G. and J.W. acknowledge support from NSF grant AST-1614569. E.C.C. was supported by an NSF Graduate Research Fellowship as well as NSF grant No. AST-1616540. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grant AST-0071048.
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