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 (OHR). 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 (Vϕ) near zero, although there is evidence for a small retrograde motion for these stars (Vϕ ∼ −15 km s−1) in the OHR. 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 OHR of the Galaxy.