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The local high-velocity tail and the Galactic escape speed

May 21, 2019
13 pages
Published in:
  • Mon.Not.Roy.Astron.Soc. 485 (2019) 3, 3514-3526
  • Published: May 21, 2019
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Abstract: (Oxford University Press)
We model the fastest moving (⁠|vtot>300 km  s−1v_{\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 (⁠|Mstar∼109 M⊙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 2r_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 |vesc(r0)=528−25+24 km  s−1v_{\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: |M200,tot=1.00−0.24+0.31×1012 M⊙M_{200, \rm tot} = 1.00^{+0.31}_{-0.24} \times 10^{12}\, \mathrm{M}_\odot|⁠, |c200=10.9−3.3+4.4c_{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 |M200,tot∼1012 M⊙M_{200, \rm tot} \sim 10^{12}\, \mathrm{M}_\odot|⁠.
  • Galaxy: fundamental parameters
  • Galaxy: kinematics and dynamics