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 MW’s stellar populations and in the general galaxy population and thus plays a critical role in galaxy evolution .