Work

The local high-velocity tail and the Galactic escape speed

Year: 2019

Type: article

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 |$M_{200, \rm tot} \sim 10^{12}\, \mathrm{M}_\odot$|⁠.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stz623