Rediscovering the Milky Way with orbit superposition approach and APOGEE data III. Panoramic view of the bulge

TL;DR

This study uses an orbit superposition method combined with Gaia and APOGEE data to map the Milky Way bulge's 3D structure, revealing its complex stellar populations, orbital families, and chemical gradients, supporting a secular evolution origin.

Contribution

It extends the orbit superposition approach to the MW bulge, integrating Gaia and APOGEE data to analyze its detailed structure and composition without observational biases.

Findings

The bulge comprises two main populations from thick and thin discs.

Multiple sub-populations are identified with distinct orbital and chemical properties.

No universal metallicity gradient characterizes the bulge, with gradients linked to its structure.

Abstract

The innermost parts of the Milky Way (MW) are very difficult to observe due to the high extinction along the line of sight, especially close to the disc mid-plane. However, this region contains the most massive complex stellar component of the MW, the bulge, primarily composed of disc stars whose structure is (re-)shaped by the evolution of the bar. In this work, we extend the application of the orbit superposition method to explore the present-day 3D structure, orbital composition, chemical abundance trends and kinematics of the MW bulge. Thanks to our approach, we are able to transfer astrometry from Gaia and stellar parameters from APOGEE DR 17 to map the inner MW without obscuration by the survey footprint and selection function. We demonstrate that the MW bulge is made of two main populations originating from a metal-poor, high-{\alpha} thick disc and a metal-rich, low-{\alpha} thin disc, with a mass ratio of 4:3, seen as two major components in the MDF. Finer MDF structures hint at multiple sub-populations associated with different orbital families of the bulge, which, however, have broad MDFs themselves. Decomposition using 2D GMMs in [Fe/H] -[Mg/Fe] identifies five components including a population with ex-situ origin. Two dominant ones correspond to the thin and thick discs and two in between trace the transition between them. We show that no universal metallicity gradient value can characterise the MW bulge. The radial gradients closely trace the X-shaped bulge density structure, while the vertical gradient variations follow the boxy component. While having, on average, subsolar metallicity, the MW bulge populations are more metal-rich compared to the surrounding disc, in agreement with extragalactic observations and state-of-the-art simulations reinforcing its secular origin.