2013MNRAS.430.2973K

Here, we study the metallicity bias in the radial and tangential velocity dispersions, the derived quantity called anisotropy and the mean azimuthal velocity profiles of the Milky Way stellar halo using Blue Horizontal Branch (BHB) stars. The comparatively metal-rich sample ([Fe/H] > -2) has prograde motion and is found to have an offset of 40kms-1 in the mean azimuthal velocity with respect to a metal-poor sample ([Fe/H] ≤ -2) which has retrograde motion. The difference in rotation between the most metal-poor and most metal-rich population was found to be around 65kms-1. For Galactocentric distances r ≲ 16kpc, an offset in velocity dispersion profiles and anisotropy can also be seen. In the inner regions, the metal-poor population is in average tangential; however, anisotropy is found to decrease monotonically with radius independent of metallicity. Beyond r = 16kpc, both the metal-rich and the metal-poor samples are found to have tangential motion. The metallicity bias in the kinematics of the halo stars qualitatively supports the co-existence of at least two-components in the halo having different formation history e.g. in situ formation and formation by accretion.