The local high-velocity tail and the Galactic escape speed

Abstract

We model the fastest moving ($v_{\rm tot} \gt 300 , {\rm km}, , {\rm s}^{-1}$) 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 ($M_{\rm star}\sim 10^9 , \mathrm{M}\odot$) 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 $v{\rm esc}(r_0) = 528^{+24}{-25} , {\rm km}, , {\rm s}^{-1}$. We use our measurement of the escape velocity to estimate the total Milky Way mass, and dark halo concentration: $M{200, \rm tot} = 1.00^{+0.31}{-0.24} \times 10^{12}, \mathrm{M}\odot$, $c_{200}=10.9^{+4.4}{-3.3}$. Our estimated mass agrees with recent results in the literature that seem to be converging on a Milky Way mass of $M{200, \rm tot} \sim 10^{12}, \mathrm{M}_\odot$.