The local high-velocity tail and the Galactic escape speed

We model the fastest moving (upsilon(tot) > 300 km s(-1)) local (D less than or similar to 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-star similar to 10(9) M-circle dot) 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 2r(200), beyond which stars can escape. After applying our methodology to the Gaia data, we find a local (r(0) = 8.3 kpc) escape speed of upsilon(esc)(r(0)) = 528(-25)(+24) km s(-1). We use our measurement of the escape velocity to estimate the total Milky Way mass, and dark halo concentration: M-200,(tot) = 1.00(-0.24)(+0.31) x 10(12) M-circle dot, c(200) = 10.9(-3.3)(+4.4). Our estimated mass agrees with recent results in the literature that seem to be converging on a Milky Way mass of M-200,(tot) similar to 10(12) M-circle dot.