The Halo Assembly in Lambda-CDM : Observations in 7 Dimensions ( HALO7D ) dataset consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope-measured proper motions of Milky Way ( MW ) halo main sequence turnoff stars in the CANDELS fields .
In this paper , the second in the HALO7D series , we present the proper motions for the HALO7D sample .
We discuss our measurement methodology , which makes use of a Bayesian mixture modeling approach for creating the stationary reference frame of distant galaxies .
Using the 3D kinematic HALO7D sample , we estimate the parameters of the halo velocity ellipsoid , \langle v _ { \phi } \rangle, \sigma _ { r } , \sigma _ { \phi } , \sigma _ { \theta } , and the velocity anisotropy \beta .
Using the full HALO7D sample , we find \beta = 0.63 \pm 0.05 at \langle r \rangle = 24 kpc .
We also estimate the ellipsoid parameters for our sample split into three apparent magnitude bins ; the posterior medians for these estimates of \beta , while consistent with one another , increase as a function of mean sample distance .
Finally , we estimate \beta in each of the individual HALO7D fields .
We find that the velocity anisotropy \beta can vary from field to field , which suggests that the halo is not phase mixed at \langle r \rangle = 24 kpc .
We explore the \beta variation across the skies of two stellar halos from the Latte suite of FIRE-2 simulations , finding that both simulated galaxies show \beta variation over a similar range to the variation observed across the four HALO7D fields .
The accretion histories of the two simulated galaxies result in different \beta variation patterns ; spatially mapping \beta is thus a way forward in characterizing the accretion history of the Galaxy .