Λ Cold Dark Matter, Stellar Feedback, and the Galactic Halo Abundance Pattern

(Abridged) The hierarchical formation scenario for the stellar halo requiresthe accretion and disruption of dwarf galaxies, yet low-metallicity halo starsare enriched in alpha-elements compared to similar, low-metallicity stars indwarf spheroidal (dSph) galaxies. We address this primary challenge for thehierarchical formation scenario for the stellar halo by combining chemicalevolution modelling with cosmologically-motivated mass accretion histories forthe Milky Way dark halo and its satellites. We demonstrate that stellar haloand dwarf galaxy abundance patterns can be explained naturally within the LCDMframework. Our solution relies fundamentally on the LCDM model prediction thatthe majority of the stars in the stellar halo were formed within a fewrelatively massive, ~5 x 10^10 Msun, dwarf irregular (dIrr)-size dark matterhalos, which were accreted and destroyed ~10 Gyr in the past. These systemsnecessarily have short-lived, rapid star formation histories, are enrichedprimarily by Type II supernovae, and host stars with enhanced [a/Fe]abundances. In contrast, dwarf spheroidal galaxies exist within low-mass darkmatter hosts of ~10^9 Msun, where supernovae winds are important in setting theintermediate [a/Fe] ratios observed. Our model includes enrichment from Type Iaand Type II supernovae as well as stellar winds, and includes aphysically-motivated supernovae feedback prescription calibrated to reproducethe local dwarf galaxy stellar mass - metallicity relation. We userepresentative examples of the type of dark matter halos we expect to host adestroyed ``stellar halo progenitor'' dwarf, a surviving dIrr, and a survivingdSph galaxy, and show that their derived abundance patterns, stellar masses,and gas masses are consistent with those observed for each type of system.Comment: 10 pages, 3 figures, version accepted by Ap