Halo substructure and Milky Way formation

The accretion history of the Milky Way is partially encoded in its halo substructure. I describe the results of a systematic statistical search for elements of cold halo substructure (ECHOS) in the radial velocity distribution of stars in the inner halo of the Milky Way observed during the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. Radial velocity substructure is systematically older than surface brightness substructure (e.g., tidal streams), and therefore provides a direct measure of the accretion history of the Milky Way in a region and time interval that has yet to be fully explored. I measure average chemical abundance properties in both ECHOS and in the kinematically smooth inner halo stellar population, and I find that ECHOS are unlikely to be disrupted globular clusters or ultrafaint dwarf spheroidal (dSph) galaxies. The chemical abundances of stars in ECHOS indicate that ECHOS are likely the debris of a Mtot ∼ 109 solar mass dSph galaxy or galaxies. Spatial autocorrelation in line of sight averaged abundance measurements in the smooth halo population indicates that the contribution of disrupted dSph galaxies to the smooth halo population increases with Galactocentric radius, becoming comparable to the classical smooth halo population at about 15 Galactocentric kpc.