Substructure at High Speed II: The Local Escape Velocity and Milky Way
  Mass with Gaia DR2
  Measuring the escape velocity of the Milky Way is critical in obtaining the
mass of the Milky Way, understanding the dark matter velocity distribution, and
building the dark matter density profile. In Necib $\&$ Lin (2021), we
introduced a strategy to robustly measure the escape velocity. Our approach
takes into account the presence of kinematic substructures by modeling the tail
of the stellar distribution with multiple components, including the stellar
halo and the debris flow called the Gaia Sausage (Enceladus). In doing so, we
can test the robustness of the escape velocity measurement for different
definitions of the "tail" of the velocity distribution, and the consistency of
the data with different underlying models. In this paper, we apply this method
to the second data release of Gaia and find that a model with at least two
components is preferred. Based on a fit with three bound components to account
for the disk, relaxed halo, and the Gaia Sausage, we find the escape velocity
of the Milky Way at the solar position to be $v_{\rm{esc}}=
484.6^{+17.8}_{-7.4}$ km/s. Assuming a Navarro-Frenck-White dark matter
profile, and taken in conjunction with a recent measurement of the circular
velocity at the solar position of $v_c = 230 \pm 10$ km/s, we find a Milky Way
concentration of $c_{200} = 13.8^{+6.0}_{-4.3}$ and a mass of $M_{200} =
7.0^{+1.9}_{-1.2} \times 10^{11} M_{\odot}$, which is considerably lighter than
previous measurements.
