The local high-velocity tail and the Galactic escape speed

We model the fastest moving (⁠v_{tot}>300km 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_{star}∼10^{9}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 2xr_{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 v_{esc}(r_{0})=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: M_{200,tot}=1.00 +0.31 −0.24 ×10^{12}M⊙⁠, c_{200}=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 M_{200,tot}∼10^{12}M⊙⁠.