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] $ 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.