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The velocity anisotropy parameter, beta, is a measure of the kinematic stateof orbits in the stellar halo which holds promise for constraining the mergerhistory of the Milky Way (MW). We determine global trends for beta as afunction of radius from three suites of simulations, including accretion onlyand cosmological hydrodynamic simulations. We find that both types ofsimulations are consistent and predict strong radial anisotropy (<beta>~0.7)for Galactocentric radii greater than 10 kpc. Previous observations of beta forthe MW's stellar halo claim a detection of an isotropic or tangential "dip" atr~20 kpc. Using the N-body+SPH simulations, we investigate the temporalpersistence, population origin, and severity of "dips" in beta. We find dips inthe in situ stellar halo are long-lived, while dips in the accreted stellarhalo are short-lived and tied to the recent accretion of satellite material. Wealso 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 angularexpanse. While all of these mechanisms are plausible drivers for the beta dipobserved in the MW, in the simulations, each mechanism has a unique metallicitysignature associated with it, implying that future spectroscopic surveys coulddistinguish between them. Since an accurate knowledge of beta(r) is requiredfor measuring the mass of the MW halo, we note significant transient dips inbeta could cause an overestimate of the halo's mass when using spherical Jeansequation modeling.

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