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We present evidence that multiple accretion events are required to explainthe origin of the $Gaia$-Sausage and Enceladus (GSE) structures, based on ananalysis of dynamical properties of main-sequence stars from the Sloan DigitalSky Survey Data Release 12 and $Gaia$ Data Release 2. GSE members are selectedto have eccentricity ($e$) $>$ 0.7 and [Fe/H] $<$ -1.0, and separated into lowand high orbital-inclination (LOI/HOI) groups. We find that the LOI starsmainly have $e < 0.9$ and are clearly separable into two groups with progradeand retrograde motions. The LOI stars exhibit prograde motions in theinner-halo region and strong retrograde motions in the outer-halo region. Weinterpret the LOI stars in these regions to be stars accreted from two massivedwarf galaxies with low-inclination prograde and retrograde orbits, affected todifferent extents by dynamical friction due to their different orbitaldirections. In contrast, the majority of the HOI stars have $e > 0.9$, andexhibit a globally symmetric distribution of rotational velocities ($V_{\rm\phi}$) near zero, although there is evidence for a small retrograde motion forthese stars ($V_{\rm \phi}$ $\sim$ -15 $\rm{km~s^{-1}}$) in the outer-haloregion. We consider these stars to be stripped from a massive dwarf galaxy on ahigh-inclination orbit. We also find that the LOI and HOI stars on highlyeccentric and tangential orbits with clear retrograde motions exhibit differentmetallicity peaks at [Fe/H] = -1.7 and -1.9, respectively, and argue that theyare associated with two low-mass dwarf galaxies accreted in the outer-haloregion of the Galaxy.

Evidence For Multiple Accretion Events in the $Gaia$-Sausage/Enceladus Structures