Linking the metallicity distribution of Galactic halo stars to the enrichment history of the universe

We compare the metallicity distribution of Galactic halo stars with three-d imensional realizations of hierarchical galaxy formation. Outflows from dwa rf galaxies enrich the intergalactic medium inhomogeneously, at a rate depe nding on the local galaxy density. Consequently, the first stars created in small early-forming galaxies are less metal-rich than the first stars form ed in more massive galaxies, which typically form later. As most halo stars are likely to originate in accreted dwarfs, while disk stars formed out of outflow-enriched gas, this scenario naturally generates a "metallicity flo or" for old disk stars, which we find to be roughly coincident with the hig her end of our predicted metallicity distribution of halo stars, in agreeme nt with observations. The broad and centrally peaked distribution of halo s tar metallicities is well reproduced in our models, with a natural dispersi on depending on the exact accretion history. Our modeling includes the impo rtant "baryonic stripping" effect of early outflows, which brush away the t enuously held gas in neighboring previrialized density perturbations. This stripping process does not significantly modify the predicted shape of the halo star metal distribution but inhibits star formation and hence the numb er of accreted stars, helping to reconcile our model with the observed tota l Galactic halo luminosity and the lack of low-luminosity local dwarf galax ies relative to N-body predictions.