Chemical Abundance Distributions of Galactic Halos and Their Satellite Systems in a ΛCDM Universe

We present a cosmologically motivated model for the hierachical formation ofthe stellar halo which includes a semi-analytic treatment of galactic chemicalenrichment coupled to numerical simulations that track the orbital evolutionand tidal disruption of satellites. A major motivating factor in thisinvestigation is the observed systematic difference between the chemicalabundances of stars in satellite galaxies and those in the Milky Way halo.Specifically, for the same [Fe/H] values, stars in neighboring satellitegalaxies display significantly lower [alpha/Fe] ratios than stars in the halo. We find that the observed chemical abundance patterns are a natural outcomeof the process of hierarchical assembly of the Galaxy. This result followsbecause the stellar halo in this context is built up from satellite galaxiesaccreted early on (more than 8-9 Gyr ago) and enriched in alpha-elementsproduced in Type II supernovae. In contrast, satellites which survive today aretypically accreted late (within the last 4-5 Gyr) with nearly solar [alpha/Fe]values as a result of contributions from both Type II and Type Ia supernovae.We use our model to investigate the abundance distribution functions (usingboth [Fe/H] and [alpha/Fe] ratios) for stars in the halo and in survivingsatellites. Our results suggest that the shapes and peaks in the abundancedistribution functions provide a direct probe of the accretion histories ofgalaxies.Comment: submitted to Ap