Results of recent observations of the Galactic bulge demand that we discard a simple picture of its formation , suggesting the presence of two stellar populations represented by two peaks of stellar metallicity distribution ( MDF ) in the bulge .
To assess this issue , we construct Galactic chemical evolution models that have been updated in two respects : First , the delay time distribution ( DTD ) of type Ia supernovae ( SNe Ia ) recently revealed by extensive SN Ia surveys is incorporated into the models .
Second , the nucleosynthesis clock , the s -processing in asymptotic giant branch ( AGB ) stars , is carefully considered in this study .
This novel model first shows that the Galaxy feature tagged by the key elements , Mg , Fe , Ba for the bulge as well as thin and thick disks is compatible with a short-delay SN Ia .
We present a successful modeling of a two-component bulge including the MDF and the evolutions of [ Mg/Fe ] and [ Ba/Mg ] , and reveal its origin as follows .
A metal-poor component ( < [ Fe/H ] > \sim -0.5 ) is formed with a relatively short timescale of \sim 1 Gyr .
These properties are identical to the thick disk ’ s characteristics in the solar vicinity .
Subsequently from its remaining gas mixed with a gas flow from the disk outside the bulge , a metal-rich component ( < [ Fe/H ] > \sim +0.3 ) is formed with a longer timescale ( \sim 4 Gyr ) together with a top-heavy initial mass function that might be identified with the thin disk component within the bulge .