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 l 2 ⟩ 1 / 2 = 138 − 26 + 43 ?> km s−1, ⟨ v b 2 ⟩ 1 / 2 = 88 − 17 + 28 km s − 1 ?> , and ⟨ v LOS 2 ⟩ 1 / 2 = 91 − 14 + 27 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 (βSN ∼ 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.