We are grateful to N. Scoville for co-writing the observing proposal and his contribution to discussions of analysis and interpretation of the data and to J. Koda and J. Ott for discussing the data analysis, looking at the data and commenting on the paper. We are grateful to A. Ciurlo, A. Ghez, M. Morris and S. Gillessen for calculating the limit on Brγ and sharing it with us and discussions, and to E. Quataert, S. Ressler and J. Lu for bringing the Brγ non-detection to our attention and discussions. We thank Y. Levin, J. Cuadra, P. Goldreich, D. Lin, J. Guillochon, S. Naoz, G. Witzel, M. Coleman, S. Philippov, S. Tremaine, A. Loeb, Z. Haiman and J. Carlstrom for discussions and comments, and the North American ALMA Science Center scientists at the National Radio Astronomy Observatory, in particular A. Remijan, C. Vlahakis, M. Lacy, S. Stierwalt, A. Moullet, E. Keller and B. Kirk for their help and advice with the observational setups and data reduction. We also thank Z. Scoville for proofreading the manuscript. E.M.M. acknowledges the Bezos Fund for providing her stipend at the Institute for Advanced Study, D. and B. Groce for encouragement and for supporting her as a Groce Fellow at Caltech; and the NRAO Student Observational Support programme for supporting two years of her graduate studies. A.P. is supported by NASA through Einstein Postdoctoral Fellowship grant number PF5-160141 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. ALMA is a partnership of the ESO (representing its member states), the NSF (USA) and the NINS (Japan), together with the NRC (Canada) and the NSC and the ASIAA (Taiwan) and the KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the ESO, the AUI/NRAO and the NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Nature thanks Andreas Burkert and Reinhard Genzel for their contribution to the peer review of this work.