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

TL;DR

This study provides evidence that the Gaia-Sausage/Enceladus structures originated from multiple accretion events involving different dwarf galaxies, revealed through analysis of stellar dynamical properties and metallicity distributions.

Contribution

It demonstrates that the Gaia-Sausage/Enceladus structures are formed by multiple accretion events from dwarf galaxies with distinct orbital inclinations and metallicities.

Findings

LOI stars show prograde and retrograde motions, indicating different accretion origins.

HOI stars have high eccentricity and symmetric velocity distribution, linked to a high-inclination dwarf galaxy.

Distinct metallicity peaks suggest multiple low-mass dwarf galaxies contributed to the structures.

Abstract

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.