The complex inner Milky Way disentangled with chemistry-kinematics

Abstract

The inner ∼ 5 kiloparsec (kpc) region of the Milky Way is complex. Establishing a picture of the formation of the Galaxy requires precise understanding of the formation of this region. Here we report a study focused on disentangling the inner Milky Way using the measured positions, velocities, and element abundance ratios of red giant stars. We demonstrate that, after removing the stellar halo, the inner Galaxy populations can be grouped into three main components based on their angular momentum: bar, disc, and a previously unreported “knot” component. The knot has a spheroidal shape and is concentrated in the inner ∼ 1.5 kpc, is comprised of stars on nearly-radial orbits, and presents the most enriched [Fe/H] element abundances of the whole Galaxy. Its chemical composition is qualitatively similar to the Galactic bar, suggestive that these two populations share a common genesis. Our results also show that the bar is more slowly rotating than previously thought, with a pattern speed of Ω_bar = 24 ± 3 kms−1 kpc−1. This new estimate suggests that the influence of the bar extends beyond the solar radius.