Demographics of the inner Milky Way

Abstract

The stellar demographics of the central Milky Way have remained somewhat unclear due to observational challenges posed by high interstellar extinction. To provide a comprehensive picture of this region, Danny Horta Darrington, Michael Petersen and Jorge Peñarrubia have conducted a population study of metal-rich red giants ([Fe/H] > –0.8) in the inner Galaxy (r < 5 kpc) from APOGEE and Gaia DR3. They identified three populations co-existing spatially: inner disk stars, the bar, and a spheroidal ‘knot’, suggesting a close connection between the inner Galaxy and the Galactic bar.

While these populations are statistically distinct in the dynamic distribution, they share similar chemical composition and age characteristics, suggesting a common star formation history. The ‘knot’, composed of old stars with a spheroidal distribution, resembles most characteristics of a classical bulge, except for its broad metallicity range from –0.8 to super-solar metallicity. The authors suggest that this discrepancy could arise from a different formation pathway for the ‘knot’: secular evolution. This is the mechanism believed to have formed the Milky Way bar; as opposed to the major merger pathway, which is hypothesized to give rise to a classical bulge.