ABSOLUTE PROPER MOTIONS TO B ～22.5: LARGE-SCALE STREAMING MOTIONS AND THE STRUCTURE AND ORIGIN OF THE GALACTIC HALO

摘要

With an enlarged sample of radial velocities, we explore the phase-space and metallicity distributions for a sample of dwarf stars with distances up to 8 kpc in a deep north Galactic pole proper motion survey. We find that the halo stars in the sample are concentrated into clumps in the combined phase-space and metallicity distribution, one of the most prominent clumps representing a retrograde, predominantly metal-poor ([Fe/H] < —0.8) stream moving toward the Galactic plane. These new data reveal that (1) the halo is not dynamically mixed but presently contains a significant fraction of stars with membership in correlated stellar streams, (2) phase-space substructure might account for differences in halo kinematics derived among surveys along different lines of sight, and (3) a significant fraction of the halo field star population may be derived from the accretion of stellar agglomerations (e.g., star clusters, satellite galaxies, or Searle & Zinn "fragments").