Rix, Hans WalterChandra, VedantZasowski, GailPillepich, AnnalisaKhoperskov, SergeyFeltzing, SofiaWyse, Rosemary F.G.Frankel, NeigeHorta, DannyKollmeier, JunaStassun, KeivanNess, Melissa K.Bird, Jonathan C.Nidever, DavidFernández-Trincado, José G.Amarante, João A.S.Laporte, Chervin F.P.Lian, Jianhui2025-05-232025-05-230004-637XORCID:/0000-0001-5082-6693/work/184099911http://www.scopus.com/inward/record.url?scp=85209572347&partnerID=8YFLogxKhttps://hdl.handle.net/1885/733752563We show with Gaia XP spectroscopy that extremely metal-rich (EMR) stars in the Milky Way ([M/H]XP ≳ 0.5) are largely confined to a tight "knot"at the center of the Galaxy. This EMR knot is round in projection, has a fairly abrupt edge near RGC,proj ∼ 1.5 kpc, and is a dynamically hot system. This central knot also contains very metalrich (VMR; +0.2 . [M/H]XP . +0.4) stars. However, in contrast to EMR stars, the bulk of VMR stars forms an extended, highly flattened distribution in the inner Galaxy (RGC ≲5 kpc). We draw on TNG50 simulations of Milky Way analogs for context and find that compact, metal-rich knots confined to ≲1.5 kpc are a universal feature. In typical simulated analogs, the top 5%-10% most metal-rich stars are confined to a central knot; however, in our Milky Way data this fraction is only 0.1%. Dust-penetrating wide-area near-infrared spectroscopy, such as the fifth Sloan Digital Sky Survey, will be needed for a rigorous estimate of the fraction of stars in the Galactic EMR knot. Why in our Milky Way only EMR giants are confined to such a central knot remains to be explained. Remarkably, the central few kiloparsecs of the Milky Way harbor both the highest concentration of metal-poor stars (the "poor old heart") and almost all EMR stars. This highlights the stellar population diversity at the bottom of galactic potential wells.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. Funding for the Sloan Digital Sky Survey V (and its earlier phases such as SDSS-IV) has been provided by the Alfred P. Sloan Foundation, the Heising-Simons Foundation, the National Science Foundation, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS website is www.sdss.org. SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration, including the Carnegie Institution for Science, Chilean National Time Allocation Committee (CNTAC) ratified researchers, the Gotham Participation Group, Harvard University, Heidelberg University, The Johns Hopkins University, L'Ecole polytechnique f\u00E9d\u00E9rale de Lausanne (EPFL), Leibniz Institut for Astrophysics Potsdam (AIP), Max Planck Institute for Astronomy (MPIA Heidelberg), Max Planck Institut for Extraterrestrial Physics (MPE), Nanjing University, National Astronomical Observatories of China (NAOC), New Mexico State University, The Ohio State University, Pennsylvania State University, Smithsonian Astrophysical Observatory, Space Telescope Science Institute (STScI), the Stellar Astrophysics Participation Group, Universidad Nacional Aut\u00F3noma de M\u00E9xico, University of Arizona, University of Colorado Boulder, University of Illinois at Urbana-Champaign, University of Toronto, University of Utah, University of Virginia, Yale University, and Yunnan University. N.F. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC, funding reference number 568580) through a CITA postdoctoral fellowship, and acknowledges partial support from an Arts & Sciences Postdoctoral Fellowship at the University of Toronto. SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration, including the Carnegie Institution for Science, Chilean National Time Allocation Committee (CNTAC) ratified researchers, the Gotham Participation Group, Harvard University, Heidelberg University, The Johns Hopkins University, L\u2019Ecole polytechnique f\u00E9d\u00E9rale de Lausanne (EPFL), Leibniz Institut for Astrophysics Potsdam (AIP), Max Planck Institute for Astronomy (MPIA Heidelberg), Max Planck Institut for Extraterrestrial Physics (MPE), Nanjing University, National Astronomical Observatories of China (NAOC), New Mexico State University, The Ohio State University, Pennsylvania State University, Smithsonian Astrophysical Observatory, Space Telescope Science Institute (STScI), the Stellar Astrophysics Participation Group, Universidad Nacional Aut\u00F3noma de M\u00E9xico, University of Arizona, University of Colorado Boulder, University of Illinois at Urbana-Champaign, University of Toronto, University of Utah, University of Virginia, Yale University, and Yunnan University. N.F. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC, funding reference number 568580) through a CITA postdoctoral fellowship, and acknowledges partial support from an Arts & Sciences Postdoctoral Fellowship at the University of Toronto.enPublisher Copyright: © 2024. The Author(s).Gaia (2360)Galaxy chemical evolution (580)Galaxy evolution (594)Metallicity (1031)Milky Way dynamics (1051)Milky Way evolution (1052)Milky Way formation (1053)2024-11-0110.3847/1538-4357/ad7aee85209572347