Evidence for Two Early Accretion Events That Built the Milky Way Stellar
  Halo
  The Gaia Sausage is the major accretion event that built the stellar halo of
the Milky Way galaxy. Here, we provide dynamical and chemical evidence for a
second substantial accretion episode, distinct from the Gaia Sausage. The
Sequoia Event provided the bulk of the high energy retrograde stars in the
stellar halo, as well as the recently discovered globular cluster FSR 1758.
There are up to 6 further globular clusters, including $\omega$~Centauri, as
well as many of the retrograde substructures in Myeong et al. (2018),
associated with the progenitor dwarf galaxy, named the Sequoia. The stellar
mass in the Sequoia galaxy is $\sim 5 \times 10^{7} M_\odot$, whilst the total
mass is $\sim 10^{10} M_\odot$, as judged from abundance matching or from the
total sum of the globular cluster mass. Although clearly less massive than the
Sausage, the Sequoia has a distinct chemo-dynamical signature. The strongly
retrograde Sequoia stars have a typical eccentricity of $\sim0.6$, whereas the
Sausage stars have no clear net rotation and move on predominantly radial
orbits. On average, the Sequoia stars have lower metallicity by $\sim 0.3$ dex
and higher abundance ratios as compared to the Sausage. We conjecture that the
Sausage and the Sequoia galaxies may have been associated and accreted at a
comparable epoch.
