tokens: Acknowledgements We are grateful to our referee Mike Bessell for his very thorough and useful report of our paper. The project is supported by fondation MERAC. N.C., H.G.L., and A.J.G. were supported by Sonderforschungsbereich SFB 881 “The Milky Way System” (subproject A4) of the German Research Foundation (DFG). R.S.K. acknowledges support from the European Research Council via the Advanced Grant “STARLIGHT: Formation of the First Stars” (project number 339177). C.A. acknowledges funding from the Alexander von Humboldt Foundation.
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