Cosmology and the Stellar Halo

Abstract

If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted and subsequently tidally destroyed ~100 low-mass galaxies in the past ~12 Gyr. We model this process using a hybrid semi-analytic plus N-body approach and show that the disrupted systems lead naturally to stellar halos with masses and density profiles much like the stellar halo of our own Galaxy. We present predictions for the properties of stellar halos and show that ours is likely dominated by substructure beyond \~50kpc and more spatially smooth within that radius. The average stellar halo density profile is expected to drop off with radius more quickly than that of the dark matter because the stellar halo is formed from the most tightly bound material in infalling systems, while the majority of the accreted dark matter is stripped and deposited at larger radii. We argue that stars associated with the inner halo should be quite different chemically from stars in surviving satellites and also different from stars in the outer halo or those liberated in recent disruption events. We discuss how deep halo surveys and chemical probes may be useful tools for uncovering evidence of accretion. Searches of this kind offer a direct test of whether cosmology is indeed hierarchical on small scales.