Kinematics and chemistry of halo substructures: The vicinity of the Virgo Overdensity

We present observations obtained with the Anglo-Australian Telescope's 2dF wide field spectrograph AAOmega of K-type stars located within a region of the sky which contains the Virgo Overdensity and the leading arm of the Sagittarius Stream. On the basis of the resulting velocity histogram, we isolate halo substructures in these overlapping regions including Sagittarius and previously discovered Virgo groups. Through comparisons with N-body models of the Galaxy-Sagittarius interaction, we find a tri-axial dark matter halo is favored and we exclude a prolate shape. This result is contradictory with other observations along the Sagittarius leading arm, which typically favor prolate models. We have also uncovered K-giant members of Sagittarius that are notably more metal-poor (〈[Fe/H]〉 = -1.7 ± 0.3dex) than previous studies. This suggests a significantly wider metallicity distribution exists in the Sagittarius Stream than formerly considered. We also present data on five carbon stars which were discovered in our sample.