The chemical properties of the Milky Way's on-bar and off-bar regions: evidence for inhomogeneous star formation history in the bulge

TL;DR

This study examines the chemical abundance distributions in the Milky Way's bar and off-bar regions, revealing inhomogeneous star formation history and vertical heating effects, which inform models of galactic evolution.

Contribution

It provides detailed chemical abundance analysis of the Milky Way's bar and disc regions, highlighting azimuthal and vertical variations linked to star formation and bar buckling.

Findings

On-bar and off-bar stars at 3-5 kpc have similar chemical distributions.

Differences observed at 2-3 kpc suggest varied star formation histories.

Vertical distribution of low-alpha stars indicates vertical heating in the bar.

Abstract

Numerous studies of integrated starlight, stellar counts, and kinematics have confirmed that the Milky Way is a barred galaxy. However, far fewer studies have investigated the bar's stellar population properties, which carry valuable independent information regarding the bar's formation history. Here we conduct a detailed analysis of chemical abundance distributions ([Fe/H] and [Mg/Fe]) in the on-bar and off-bar regions to study the azimuthal variation of star formation history (SFH) in the inner Galaxy. We find that the on-bar and off-bar stars at Galactocentric radii 3 $< r_{\rm GC}<$ 5 kpc have remarkably consistent [Fe/H] and [Mg/Fe] distribution functions and [Mg/Fe]--[Fe/H] relation, suggesting a common SFH shared by the long bar and the disc. In contrast, the bar and disc at smaller radii (2 $< r_{\rm GC} <$ 3 kpc) show noticeable differences, with relatively more very metal-rich ([Fe/H]~0.4) stars but fewer solar abundance stars in the bar. Given the three-phase star formation history proposed for the inner Galaxy in Lian et al. (2020b), these differences could be explained by the off-bar disc having experienced either a faster early quenching process or recent metal-poor gas accretion. Vertical variations of the abundance distributions at small $r_{\rm GC}$ suggest a wider vertical distribution of low-$\alpha$ stars in the bar, which may serve as chemical evidence for vertical heating through the bar buckling process. The lack of such vertical variations outside the bulge may then suggest a lack of vertical heating in the long bar.