Beta Dips in the Gaia Era: Simulation Predictions of the Galactic
  Velocity Anisotropy Parameter 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 both types of
simulations are consistent and predict strong radial anisotropy (<beta>~0.7)
for Galactocentric radii greater than 10 kpc. Previous observations of beta for
the MW's stellar halo claim a detection of an isotropic or tangential "dip" at
r~20 kpc. Using the N-body+SPH simulations, we investigate the temporal
persistence, population origin, and severity of "dips" in beta. We find 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~1 can result in a present day low
(isotropic to tangential) value of beta over a wide range of radii and angular
expanse. While all of these mechanisms are plausible drivers for the beta dip
observed in the MW, in the simulations, each mechanism 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 significant transient dips in
beta could cause an overestimate of the halo's mass when using spherical Jeans
equation modeling.
