The Milky Way's bulge star formation history as constrained from its bimodal chemical abundance distribution

Jianhui Lian, Gail Zasowski, Sten Hasselquist, David M. Nataf, Daniel Thomas, Christian Moni Bidin, José G. Fernández-Trincado, D. A. Garcia-Hernandez, Richard R. Lane, Steven R. Majewski, Alexandre Roman-Lopes, Mathias Schultheis

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We conduct a quantitative analysis of the star formation history (SFH) of the Milky Way’s (MW) bulge by exploiting the constraining power of its stellar [Fe/H] and [Mg/Fe] distribution functions. Using Apache Point Observatory Galactic Evolution Experiment survey data, we confirm the previously established bimodal [Mg/Fe]–[Fe/H] distribution within 3 kpc of the inner Galaxy. To fit the chemical bimodal distribution, we use a simple but flexible star formation framework, which assumes two distinct stages of gas accretion and star formation, and systematically evaluate a wide multidimensional parameter space. We find that the data favour a three-phase SFH that consists of an initial starburst, followed by a rapid star formation quenching episode, and a lengthy, quiescent secular evolution phase. The metal-poor, high-α bulge stars ([Fe/H] < 0.0 and [Mg/Fe] > 0.15) are formed rapidly (<2 Gyr) during the early starburst. The density gap between the high- and low-α sequences is due to the quenching process. The metal-rich, low-α population ([Fe/H] > 0.0 and [Mg/Fe] < 0.15) then accumulates gradually through inefficient star formation during the secular phase. This is qualitatively consistent with the early SFH of the inner disc. Given this scenario, a notable fraction of young stars (age <5 Gyr) is expected to persist in the bulge. Combined with extragalactic observations, these results suggest that a rapid star formation quenching process is responsible for bimodal distributions in both the MW’s stellar populations and in the general galaxy population and thus plays a critical role in galaxy evolution.
Original languageEnglish
Pages (from-to)3557-3570
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date27 Jul 2020
Publication statusPublished - 1 Sept 2020


  • astro-ph.GA
  • astro-ph.SR
  • galaxy: abundances
  • galaxy: bulge
  • galaxy: evolution
  • galaxy: formation
  • galaxy: stellar content
  • galaxy: structure


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