Evidence For Multiple Accretion Events in the $Gaia$-Sausage/Enceladus
  Structures
  We present evidence that multiple accretion events are required to explain
the origin of the $Gaia$-Sausage and Enceladus (GSE) structures, based on an
analysis of dynamical properties of main-sequence stars from the Sloan Digital
Sky Survey Data Release 12 and $Gaia$ Data Release 2. GSE members are selected
to have eccentricity ($e$) $>$ 0.7 and [Fe/H] $<$ -1.0, and separated into low
and high orbital-inclination (LOI/HOI) groups. We find that the LOI stars
mainly have $e < 0.9$ and are clearly separable into two groups with prograde
and retrograde motions. The LOI stars exhibit prograde motions in the
inner-halo region and strong retrograde motions in the outer-halo region. We
interpret the LOI stars in these regions to be stars accreted from two massive
dwarf galaxies with low-inclination prograde and retrograde orbits, affected to
different extents by dynamical friction due to their different orbital
directions. In contrast, the majority of the HOI stars have $e > 0.9$, and
exhibit a globally symmetric distribution of rotational velocities ($V_{\rm
\phi}$) near zero, although there is evidence for a small retrograde motion for
these stars ($V_{\rm \phi}$ $\sim$ -15 $\rm{km~s^{-1}}$) in the outer-halo
region. We consider these stars to be stripped from a massive dwarf galaxy on a
high-inclination orbit. We also find that the LOI and HOI stars on highly
eccentric and tangential orbits with clear retrograde motions exhibit different
metallicity peaks at [Fe/H] = -1.7 and -1.9, respectively, and argue that they
are associated with two low-mass dwarf galaxies accreted in the outer-halo
region of the Galaxy.
