Speaker: Xiangxiang Xue (NAOC)
Title: Mapping the Milky Way’s Dark Matter and Stellar Halo
Language: Chinese
Time: March  24, 2011 (Thursday), 2PM
Location: Middle conf. room, 3rd floor

Abstract:
Through spectroscopy of about 4600 BHB stars in the Milky Way’s halo,SDSS has provided an unprecedented set of distant dynamical tracers (with radial velocities and precise distances) to map the mass distribution of our Galaxy’s dark matter halo. We present a rigorous analysis of these data, which imply that the MW’s dark matter halo has a virial mass of only 1 x 10^12Msun, with a likely error on the mass enclosed within 60 kpc of only 20%. This is somewhat lighter than many previous estimates and implies that the Milky Way was exceptionally efficient at forming stars.

At the same time the BHB stars can serve as an excellent set of kinematic tracers to look for velocity substructure in the stellar halo, a signpost of hierarchical assembly. Using a cumulative "close pair distribution" (CPD) as a statistic in the 4-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical signi cance among the BHB stars. Furthermore, BHB stars located in the outer halo, beyond 20 kpc from the Galactic center, exhibit statistically stronger substructure signatures than at r<20 kpc. However, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population.