We thank the anonymous referee for the many useful comments that helped us improve this manuscript. The followup DECam observations taken on 2023 June were obtained during a time trade with the Dark Energy Camera Legacy Survey (DECaLS) team. We note that Leo VI was also independently identified as a possible UFD among ∼300 candidates reported in an unpublished search of PS1 data by C. J. Grillmair (2018). C.Y.T. was supported by the U.S. National Science Foundation (NSF) through the grants AST-2108168 and AST-2307126. W.C. thanks Michael Lundquist and the staff of the W.M. Keck Observatory for extensive assistance on the night of our Keck observations, and acknowledges support from a Gruber Science Fellowship at Yale University. A.B.P. acknowledges support from NSF grant AST-1813881. J.A.C.-B. acknowledges support from FONDECYT Regular N 1220083. C.E.M.-V. is supported by the international Gemini Observatory, a program of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation, on behalf of the Gemini partnership of Argentina, Brazil, Canada, Chile, the Republic of Korea, and the United States of America. G.E. M. acknowledges support from the University of Toronto Arts & Science Postdoctoral Fellowship program, the Dunlap Institute, and the Natural Sciences and Engineering Research Council of Canada (NSERC) through grant RGPIN-2022-04794. D.J.S. acknowledges support from NSF grant AST-2205863. The DELVE project is partially supported by Fermilab LDRD project L2019-011, the NASA Fermi Guest Investigator Program Cycle 9 grant 91201, and the U.S. National Science Foundation (NSF) under grants AST-2108168 and AST2307126. This work is supported by the Fermilab Visiting Scholars Award Program from the Universities Research Association. This material is also based upon work supported by the National Science Foundation Graduate Research Fellowship Program under grant No. DGE2139841. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. Funding for the DES Projects has been provided by the US Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute for Cosmological Physics at the University of Chicago, Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy Survey. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Enérgeticas, Medioambientales y Tecnológicas–Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciències de l'Espai (IEEC/CSIC), the Institut de Física d'Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universität München and the associated Excellence Cluster Universe, the University of Michigan, NSF NOIRLab, the University of Nottingham, the Ohio State University, the OzDES Membership Consortium, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University. This work was possible based on observations at NSF Cerro Tololo Inter-American Observatory, NSF NOIRLab (NOIRLab Prop. ID 2019A-0305; PI: Alex Drlica-Wagner), which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/ gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/ dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work has made use of the Local Volume Database (https://github.com/apace7/local_volume_database; Pace 2024). This manuscript has been authored by Fermi Research Alliance, LLC, under contract No. DE-AC02-07CH11359 with the US Department of Energy, Office of Science, Office of High Energy Physics. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paidup, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes

We report the discovery and spectroscopic confirmation of an ultra-faint Milky Way satellite in the constellation of Leo. This system was discovered as a spatial overdensity of resolved stars observed with Dark Energy Camera (DECam) data from an early version of the third data release of the DECam Local Volume Exploration (or DELVE) survey. The low luminosity ( ; ), large size ( pc), and large heliocentric distance ( kpc) are all consistent with the population of ultra-faint dwarf galaxies (UFDs). Using Keck/DEIMOS observations of the system, we were able to spectroscopically confirm nine member stars, while measuring a tentative mass-to-light ratio of and a nonzero metallicity dispersion of , further confirming Leo VI's identity as a UFD. While the system has a highly elliptical shape, , we do not find any conclusive evidence that it is tidally disrupting. Moreover, despite the apparent on-sky proximity of Leo VI to members of the proposed Crater-Leo infall group, its smaller heliocentric distance and inconsistent position in energy–angular momentum space make it unlikely that Leo VI is part of the proposed infall group. © 2025. The Author(s)

Tan, C. Y. et al.-- Full list of authors: Tan, C. Y.; Cerny, W.; Drlica-Wagner, A.; Pace, A. B.; Geha, M.; Ji, A. P.; Li, T. S.; Adamów, M.; Anbajagane, D.; Bom, C. R.; Carballo-Bello, J. A.; Carlin, J. L.; Chang, C.; Chaturvedi, A.; Chiti, A.; Choi, Y.; Collins, M. L. M.; Doliva-Dolinsky, A.; Ferguson, P. S.; Gruendl, R. A.; James, D. J.; Limberg, G.; Navabi, M.; Martínez-Delgado, D.; Martínez-Vázquez, C. E.; Medina, G. E.; Mutlu-Pakdil, B.; Nidever, D. L.; Noël, N. E. D.; Riley, A. H.; Sakowska, J. D.; Sand, D. J.; Sharp, J.; Stringfellow, G. S.; Tolley, C.; Tucker, D. L.; Vivas, A. K.; Delve Collaboration

Peer reviewed