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 = 1382(-26)(+43) km s(-1), < v(b)(2)>(1/2) = 88 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 beta using 3D kinematics outside of the solar neighborhood. We find beta = -0.3(-0.9)(+0.4), consistent with isotropy and lower than solar neighborhood beta measurements by 2 sigma (beta(SN) similar to 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 beta = -0.1(-1.0)(+0.4) which is still lower than solar neighborhood measurements by 1 sigma. The potential decrease in beta 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 similar to 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.