Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31

We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions (PMs). The PMs were measured using long baseline (5-7 years) multi-epoch Hubble Space Telescope/Advanced Camera for Surveys photometry, and the LOS velocities were extracted from deep (5-6 hr integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic (l, b, LOS) coordinate system are {< {v}_l²> }^1/2={138}_-26⁺⁴³ km s^-1, {< {v}_b²> }^1/2={88}_-17⁺²⁸ {\text{km s}}^-1, and {< {v}_{{LOS}}²> }^1/2={91}_-14⁺²⁷ {\text{km s}}^-1. We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter β using 3D kinematics outside of the solar neighborhood. We find β =-{0.3}_-0.9^+0.4, consistent with isotropy and lower than solar neighborhood β measurements by 2σ (β_SN ∼ 0.5-0.7). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to β ={0.1}_-1.0^+0.4, which is still lower than solar neighborhood measurements by 1σ. The potential decrease in β with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at r ∼ 25 kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program.