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