Tracing Galaxy Formation with Stellar Halos. I. Methods

If the favored hierarchical cosmological model is correct, then the Milky Waysystem should have accreted ~100-200 luminous satellite galaxies in the past\~12 Gyr. We model this process using a hybrid semi-analytic plus N-bodyapproach which distinguishes explicitly between the evolution of light and darkmatter in accreted satellites. This distinction is essential to our ability toproduce a realistic stellar halo, with mass and density profile much like thatof our own Galaxy, and a surviving satellite population that matches theobserved number counts and structural parameter distributions of the satellitegalaxies of the Milky Way. Our model stellar halos have density profiles whichtypically drop off with radius faster than those of the dark matter. They areassembled from the inside out, with the majority of mass (~80%) coming from the\~15 most massive accretion events. The satellites that contribute to thestellar halo have median accretion times of ~9 Gyr in the past, while survivingsatellite systems have median accretion times of ~5 Gyr in the past. Thisimplies that stars associated with the inner halo should be quite differentchemically from stars in surviving satellites and also from stars in the outerhalo or those liberated in recent disruption events. We briefly discuss theexpected spatial structure and phase space structure for halos formed in thismanner. Searches for this type of structure offer a direct test of whethercosmology is indeed hierarchical on small scales.Comment: 22 pages, 16 figures, submitted to Ap