The local high-velocity tail and the Galactic escape speed

Abstract

We model the fastest moving (⁠v tot >300kms −1 vtot>300kms−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 ⊙ Mstar∼109M⊙ ⁠) 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 kms −1 vesc(r0)=528−25+24kms−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 ⊙ M200,tot=1.00−0.24+0.31×1012M⊙ ⁠, c 200 =10.9 +4.4 −3.3 c200=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 ∼10 12 M ⊙ M200,tot∼1012M⊙ ⁠.