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

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  • Jianhui Lian
  • Gail Zasowski
  • Sten Hasselquist
  • Dr Justus Neumann
  • Steven R. Majewski
  • Roger E. Cohen
  • José G. Fernández-Trincado
  • Richard R. Lane
  • Peélope Longa-Peña
  • Alexandre Roman-Lopes
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 < rGC < 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 < rGC < 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. (2020c), 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 rGC suggest a wider vertical distribution of low-α 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.
Original languageEnglish
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Early online date24 Oct 2020
Publication statusEarly online - 24 Oct 2020


  • Lian_et_al_2020_AAM

    Rights statement: This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review. Jianhui Lian et al., The version of record 'The chemical properties of the Milky Way’s on-bar and off-bar regions: evidence for inhomogeneous star formation history in the bulge,' Monthly Notices of the Royal Astronomical Society, ahead of print, staa3256, is available online at:

    Accepted author manuscript (Post-print), 1.13 MB, PDF document

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