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 R _GC,proj ∼ 1.5 kpc, and is a dynamically hot system. This central knot also contains very metal-rich (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 (R _GC ≲ 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.