The Extremely Metal-rich Knot of Stars at the Heart of the Galaxy

We 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.