Galactic chemical abundance distributions in a ACDM universe

Observations suggest systematic differences between chemical abundances of stars in satellite galaxies and those in the Milky Way halo. Specifically, for the same [Fe/H] values, stars in surviving satellite galaxies display significantly lower [α/Fe] ratios than stars in the stellar halo. Here we investigate whether the observed differences can be explained in the framework of hierarchical structure formation. We model the chemical enrichment of a typical Milky Way galaxy in a ACDM Universe using, in combination, i) a semi-analytical code and numerical simulations that model the accretion and disruption of halo substructure and ii) a chemical evolution model that takes into account each satellite's star formation, metal enrichment and stellar feedback. Our results suggest that the observed chemical abundance patterns are a natural outcome in the process of hierarchical assembly of the Galaxy. We find that the stellar halo is built up from satellite galaxies accreted early on (more than 8-9 Gyr ago) and enriched in α-elements produced in Type II supernovae (average [α/Fe] values between 0.2-0.5). In contrast, satellites which survive today were typically accreted late (within the last 4-5 Gyr) and had at the time of accretion nearly solar [α/Fe] values as a result of the longer contribution of Type la supernovae.