  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]>~-0.5) is formed with a relatively short
timescale of ~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]>~+0.3) is formed with a longer timescale (~4 Gyr) together with a
top-heavy initial mass function that might be identified with the thin disk
component within the bulge.

Astrophysics of Galaxies