QUANTIFYING KINEMATIC SUBSTRUCTURE IN THE MILKY WAY'S STELLAR HALO

Abstract

We present and analyze the positions, distances, and radial velocities for\nover 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn\nfrom SDSS DR8. We search for position-velocity substructure in these data, a\nsignature of the hierarchical assembly of the stellar halo. Using a cumulative\n"close pair distribution" (CPD) as a statistic in the 4-dimensional space of\nsky position, distance, and velocity, we quantify the presence of\nposition-velocity substructure at high statistical significance among the BHB\nstars: pairs of BHB stars that are close in position on the sky tend to have\nmore similar distances and radial velocities compared to a random sampling of\nthese overall distributions. We make analogous mock-observations of 11\nnumerical halo formation simulations, in which the stellar halo is entirely\ncomposed of disrupted satellite debris, and find a level of substructure\ncomparable to that seen in the actually observed BHB star sample. This result\nquantitatively confirms the hierarchical build-up of the stellar halo through a\nsignature in phase (position-velocity) space. In detail, the structure present\nin the BHB stars is somewhat less prominent than that seen in most simulated\nhalos, quite possibly because BHB stars represent an older sub-population. BHB\nstars located beyond 20 kpc from the Galactic center exhibit stronger\nsubstructure than at $\\rm r_{gc} < 20$ kpc.\n