Stellar Velocity Dispersion and Anisotropy of the Milky Way Inner Halo

TL;DR

This study measures the velocity dispersion and anisotropy of stars in the Milky Way's inner halo, revealing a significant break likely caused by stellar streams, and estimates the galaxy's mass within 12 kpc.

Contribution

It provides new measurements of stellar velocity components using a combined dataset and introduces a statistical method assuming normal velocity distributions for halo analysis.

Findings

Velocity ellipsoid aligns with a spherical potential

Mass within 12 kpc is approximately 1.3×10^{11} solar masses

Detected a significant anisotropy break between 15 and 25 kpc

Abstract

We measure the three components of velocity dispersion, $\sigma_{R},\sigma_{\theta},\sigma_{\phi}$, for stars within 6 < R < 30 kpc of the Milky Way using a new radial velocity sample from the MMT telescope. We combine our measurements with previously published data so that we can more finely sample the stellar halo. We use a maximum likelihood statistical method for estimating mean velocities, dispersions, and covariances assuming only that velocities are normally distributed. The alignment of the velocity ellipsoid is consistent with a spherically symmetric gravitational potential. From the spherical Jeans equation, the mass of the Milky Way is M(<12 kpc) = $1.3\times10^{11}$ M$_{\odot}$ with an uncertainty of 40%. We also find a region of discontinuity, 15 < R < 25 kpc, where the estimated velocity dispersions and anisotropies diverge from their anticipated values, confirming at high significance the break observed by others. We argue that this break in anisotropy is physically explained by coherent stellar velocity structure in the halo, such as the Sgr stream. To significantly improve our understanding of halo kinematics will require combining radial velocities with future Gaia proper motions.