The local high-velocity tail and the Galactic escape speed

We model the fastest moving (v ₓ₎ₓ 300 \, km\, \, s^-1) local (D ≲ 3 kpc) halo stars using cosmological simulations and six-dimensional Gaia data. Our approach is to use our knowledge of the assembly history and phase-space distribution of halo stars to constrain the form of the high-velocity tail of the stellar halo. Using simple analytical models and cosmological simulations, we find that the shape of the high-velocity tail is strongly dependent on the velocity anisotropy and number density profile of the halo stars – highly eccentric orbits and/or shallow density profiles have more extended high-velocity tails. The halo stars in the solar vicinity are known to have a strongly radial velocity anisotropy, and it has recently been shown the origin of these highly eccentric orbits is the early accretion of a massive (M ₒₓ₀ₑ 10⁹ \, M_) dwarf satellite. We use this knowledge to construct a prior on the shape of the high-velocity tail. Moreover, we use the simulations to define an appropriate outer boundary of 2r200, beyond which stars can escape. After applying our methodology to the Gaia data, we find a local (r0 = 8. 3 kpc) escape speed of v ₄ₒ₂ (r₀) = 528^+24-₂₅ \, km\, \, s^-1. We use our measurement of the escape velocity to estimate the total Milky Way mass, and dark halo concentration: M₂₀₀, ₓ₎ₓ = 1. 00^+0. 31-₀. ₂₄ 10^12\, M_, c₂₀₀=10. 9^+4. 4-₃. ₃. Our estimated mass agrees with recent results in the literature that seem to be converging on a Milky Way mass of M₂₀₀, ₓ₎ₓ 10^12\, M_.