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

TL;DR

This study reconstructs the Milky Way's bulge star formation history using chemical abundance data, revealing a three-phase process with an early burst, quenching, and secular evolution, consistent with galaxy evolution theories.

Contribution

It introduces a detailed, multi-phase star formation model constrained by APOGEE chemical data, highlighting the importance of quenching in bulge formation.

Findings

Bimodal [Mg/Fe]--[Fe/H] distribution confirmed within 3 kpc of the Galactic center.

Bulge stars formed through a three-phase process: initial burst, quenching, and secular evolution.

Presence of young stars (<5 Gyr) suggests ongoing or recent star formation in the bulge.

Abstract

We conduct a quantitative analysis of the star formation history (SFH) of the Milky Way's bulge by exploiting the constraining power of its stellar [Fe/H] and [Mg/Fe] distribution functions. Using APOGEE data, we confirm the previously-established bimodal [Mg/Fe]--[Fe/H] distribution within 3 kpc of the inner Galaxy. Compared to that in the solar vicinity, the high-$\alpha$ population in the bulge peaks at a lower [Fe/H]. To fit these observations, 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 multi-dimensional parameter space. We find that the data favor 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-$\alpha$ 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-$\alpha$ sequences is due to the quenching process. The metal-rich, low-$\alpha$ 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 disk. 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 Milky Way's stellar populations and in the general galaxy population and thus plays a critical role in galaxy evolution.