The Extremely Metal Rich Knot of Stars at the Heart of the Galaxy

We show with Gaia XP spectroscopy that extremely metal-rich stars in theMilky Way (EMR; [M/H]_XP > 0.5) - but only those - are largely confined toa tight "knot" at the center of the Galaxy. This EMR knot is round inprojection, has a fairly abrupt edge near ∼ 1.5kpc, and is a dynamicallyhot 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 VMRstars form an extended, highly flattened distribution in the inner Galaxy(R_GC≲ 5 kpc). We draw on TNG50 simulations of Milky Wayanalogs for context and find that compact, metal-rich knots confined to<1.5kpc are a universal feature. In typical simulated analogs, the top 5-10most metal-rich stars are confined to a central knot; however, in our Milky Waydata this fraction is only 0.1spectroscopy, such as SDSS-V, will be needed for a rigorous estimate of thefraction of stars in the Galactic EMR knot. Why in our Milky Way only EMRgiants are confined to such a central knot remains to be explained. Remarkably,the central few kiloparsecs of the Milky Way harbor both the highestconcentration of metal-poor stars (the `poor old heart') and almost all EMRstars. This highlights the stellar population diversity at the bottom ofgalactic potential wells.