Reliability of the Measured Velocity Anisotropy of the Milky Way Stellar Halo.

Determining the velocity distribution of halo stars is essential for estimating the mass of the Milky Way and for inferring its formation history. Since the stellar halo is a dynamically hot system, the velocity distribution of halo stars is well described by the three-dimensional velocity dispersions or by the velocity anisotropy parameter . Direct measurements of consistently suggest β = 0.5–0.7 for nearby halo stars. In contrast, the value of β at large Galactocentric radius r is still controversial, since reliable proper motion data are available for only a handful of stars. In the last decade, several authors have tried to estimate β for distant halo stars by fitting the observed line-of-sight velocities at each radius with simple velocity distribution models (local fitting methods). Some results of local fitting methods imply at , which is inconsistent with recent predictions from cosmological simulations. Here we perform mock-catalog analyses to show that the estimates of β based on local fitting methods are reliable only at with the current sample size (∼10³ stars at a given radius). As r increases, the line-of-sight velocity (corrected for the solar reflex motion) becomes increasingly closer to the Galactocentric radial velocity, so it becomes increasingly more difficult to estimate the tangential velocity dispersion from the line-of-sight velocity distribution. Our results suggest that the forthcoming Gaia data will be crucial for understanding the velocity distribution of halo stars at . [ABSTRACT FROM AUTHOR]