HALO7D II: The Halo Velocity Ellipsoid and Velocity Anisotropy with Distant Main Sequence Stars

TL;DR

This paper measures the velocity anisotropy of distant Milky Way halo stars using HALO7D data, revealing field-to-field variations that suggest the halo is not phase mixed and providing insights into the Galaxy's accretion history.

Contribution

It introduces a Bayesian method for measuring halo star proper motions and estimates the velocity anisotropy and ellipsoid parameters, highlighting spatial variations and their implications.

Findings

Velocity anisotropy $eta$ is approximately 0.63 at 24 kpc.

$eta$ varies across different fields, indicating incomplete phase mixing.

Simulated halos show similar $eta$ variation, linking it to accretion history.

Abstract

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 \textit{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.