Structure of the Galactic Stellar Halo Prior to Disk Formation

We develop a method for recovering the global density distribution of theancient Galactic stellar halo prior to disk formation, based on the presentorbits of metal-poor stars observed in the solar neighborhood. The methodrelies on the adiabatic invariance of the action integrals of motion for thehalo population during the slow accumulation of a disk component, subsequent toearlier halo formation. The method is then applied to a sample of local starswith [Fe/H]<=-1.5, likely to be dominated by the halo component, taken fromBeers et al.'s recently revised and supplemented catalog of metal-poor starsselected without kinematic bias. We find that even if the Galactic potential ismade spherical by removing the disk component in an adiabatic manner, the halodensity distribution in the inner halo region (R <= 15 kpc) remains moderatelyflattened, with axial ratio of about 0.8 for stars in the abundance range[Fe/H]<=-1.8 and about 0.7 for the more metal-rich interval -1.8<[Fe/H]<=-1.5.The outer halo remains spherical for both abundance intervals. We also findthat this initial flattening of the inner halo is caused by the anisotropicvelocity dispersions of the halo stars. These results suggest that thetwo-component nature of the present-day stellar halo, characterized by a highlyflattened inner halo and nearly spherical outer halo, is a consequence of bothan initially two-component density distribution of the halo (perhaps asignature of dissipative halo formation) and of the adiabatic flattening of theinner part by later disk formation. Further implications of our results for theformation of the Galaxy are also discussed, in particular in the context of thehierarchical clustering scenario of galaxy formation.