2024/06/03 by Hans‐Walter Rix, Vedant Chandra, Rix, Hans-Walter +33 · 2 citations
Physics and Astronomy · #History and Developments in Astronomy #Historical Astronomy and Related Studies
paper · pdf · doi:10.48550/arxiv.2406.01706
We show with Gaia XP spectroscopy that extremely metal-rich stars in the Milky Way (EMR; [M/H]XP > 0.5) - but only those - 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 ∼ 1.5kpc, 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 form an extended, highly flattened distribution in the inner Galaxy (RGC\lesssim 5 kpc). We draw on TNG50 simulations of Milky Way analogs for context and find that compact, metal-rich knots confined to <1.5kpc 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 SDSS-V, 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.