The Stellar Accretion History of the Milky Way Through SEGUE Observations of Halo Substructure

We identify 20 (17 for the first time kinematically cold streams in the inner halo of the Milky Way. Our result is based on the observed spatial and radial velocity distribution of metal-poor main sequence turn-off (MPMSTO) stars in 117 Sloan Extension for Galactic Understanding (SEGUE) lines-of-sight. We show that the observed distribution is consistent with a smooth stellar component of the Milky Way's halo at large scales, but disagrees significantly at the radial velocities that correspond to our detections. We prove that all of our detections are significant to more than 10-sigma and that we expect only one of our detections to be a false-positive. These cold streams represent the observable stellar populations of those dark matter halos that merged to form the Milky Way, and we use our detections and estimates of our completeness to predict that approximately 3000 streams are present in the entire inner halo. We compare our detections with the very high resolution dark matter-only Via Lactea simulation to constrain the mapping of dark matter halos to their stellar content. Finally, we consider the implications of our detections in the context of the formation of Milky Way.

Kevin C. Schlaufman (Astronomy and Astrophysics Department, University of California, Santa Cruz)