Type Ia supernovae in a hierarchical galaxy formation model: the Milky Way

We investigate chemical evolution in Milky Way-like galaxies based on the cold dark matter model in which cosmic structures form via hierarchical merging. We introduce chemical enrichment due to type Ia supernovae (SNe Ia) into the Mitaka semi-analytic galaxy formation model developed by Nagashima & Yoshii. For the first time we treat chemical enrichment due to SNe Ia in a hierarchical galaxy formation model consistently. As a first attempt, we assume all SNe Ia to have the same lifetime, and assume instantaneous recycling for type II supernovae (SNe II). We find that our model reproduce well the metal abundance ratio [O/Fe] against [Fe/H] and the frequency distribution of iron in the solar neighbourhood. This means that the so-called G-dwarf problem is fully resolved by the hierarchical formation of galaxies. While observations of bulge stars have still uncertainties even for iron, and there are few data for oxygen, our prediction for bulge stars also well agree with those observations. More data in number and accuracy will provide independent and important constraints on galaxy formation. Furthermore, to understand better the chemical enrichment due to SNe Ia in hierarchical galaxy formation, we discuss how physical processes affect the metal abundance ratio by varying the lifetime of SNe Ia, the star formation timescale and the strength of supernova feedback.