The local high-velocity tail and the Galactic escape speed

Abstract

We model the fastest moving (v_tot > 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_star∼ 10^9 M_\odot) 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_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_\odot, 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_\odot.