The flattening of the galactic spheroid

In the solar neighborhood, the velocity dispersion of metal-poor halo stars is much smaller perpendicular to the galactic plane than in other directions. This dispersion component appears to remain small in distant samples near the galactic poles. A simple argument based on the tensor virial theorem shows that this situation cannot be globally valid for any population with the modest flattening usually assigned to metal-poor Population II. Either the observed kinematic samples are drawn from a highly flattened population, or, more probably, the kinematics of the regions sampled are atypical of the population as a whole.