Beta Dips in the Gaia Era: Simulation Predictions of the Galactic Velocity Anisotropy Parameter (β) for Stellar Halos

The velocity anisotropy parameter, beta, is a measure of the kinematic state of orbits in the stellar halo, which holds promise for constraining the merger history of the Milky Way (MW). We determine global trends for beta as a function of radius from three suites of simulations, including accretion-only and cosmological hydrodynamic simulations. We find that the two types of simulations are consistent and predict strong radial anisotropy ( similar to 0.7) for Galactocentric radii greater than 10 kpc. Previous observations of ss for the MW's stellar halo claim a detection of an isotropic or tangential "dip" at r similar to 20 kpc. Using the N-body+ SPH simulations, we investigate the temporal persistence, population origin, and severity of "dips" in beta. We find that dips in the in situ stellar halo are long-lived, while dips in the accreted stellar halo are short-lived and tied to the recent accretion of satellite material. We also find that a major merger as early as z similar to 1 can result in a present-day low (isotropic to tangential) value of beta over a broad range of radii and angles. While all of these mechanisms are plausible drivers for the beta dip observed in the MW, each mechanism in the simulations has a unique metallicity signature associated with it, implying that future spectroscopic surveys could distinguish between them. Since an accurate knowledge of beta(r) is required for measuring the mass of the MW halo, we note that significant transient dips in beta could cause an overestimate of the halo's mass when using spherical Jeans equation modeling.