The Milky Way's middle-aged inner ring

TL;DR

This study reveals an inner ring structure in the Milky Way's inner Galaxy, characterized by specific orbital, metallicity, and age properties, bridging the bar and corotation regions with implications for Galactic evolution.

Contribution

It provides the first detailed orbital and chemical characterization of the Milky Way's inner ring, linking stellar dynamics with metallicity and age distributions.

Findings

Inner ring is radially thick, vertically thin, and elongated.

Stars in the ring have an average age of ~6 Gyr and are metal-rich.

Large L4 Lagrange orbits have high metallicity and intermediate ages.

Abstract

We investigate the metallicity, age, and orbital anatomy of the inner Milky Way, specifically focussing on the outer bar region. We integrated a sample of APOGEE DR16 inner Galaxy stars in a state of the art bar-bulge potential with a slow pattern speed and investigated the link between the resulting orbits and their [Fe/H] and ages. By superimposing the orbits, we built density, [Fe/H], and age maps of the inner Milky Way, which we divided further using the orbital parameters eccentricity, |Xmax|, and |Zmax|. We find that at low heights from the Galactic plane, the Galactic bar gradually transitions into a radially thick, vertically thin, elongated inner ring with average solar [Fe/H]. This inner ring is mainly composed of stars with AstroNN ages between 4 and 9 Gyr with a peak in age between 6 and 8 Gyr, making the average age of the ring ~6 Gyr. The vertical thickness of the ring decreases markedly towards younger ages. We also find very large L4 Lagrange orbits that have average solar to super-solar metallicities and intermediate ages. Lastly, we confirm a clear X-shape in the [Fe/H] and density distributions at large Galactic heights. The orbital structure obtained for the APOGEE stars reveals that the Milky Way hosts an inner ring-like structure between the planar bar and corotation. This structure is on average metal rich, intermediately aged, and enhances the horizontal metallicity gradient along the bar's major axis.