The Unmixed Debris of Gaia-Sausage/Enceladus in the Form of a Pair of Halo Stellar Overdensities

TL;DR

This paper demonstrates that the Hercules-Aquila Cloud and Virgo Overdensity are remnants of the Gaia-Sausage/Enceladus merger, sharing chemodynamical properties and confirming their common origin in the Milky Way's history.

Contribution

It provides the first comprehensive chemodynamical analysis linking these halo overdensities directly to the Gaia-Sausage/Enceladus merger event.

Findings

HAC and VOD have similar orbital eccentricity distributions.

They share metallicity distribution functions with GSE.

Evidence confirms GSE as the main progenitor of these overdensities.

Abstract

In the first billion years after its formation, the Galaxy underwent several mergers with dwarf satellites of various masses. The debris of Gaia-Sausage/Enceladus (GSE), the galaxy responsible for the last significant merger of the Milky Way, dominates the inner halo and has been suggested to be the progenitor of both the Hercules-Aquila Cloud (HAC) and Virgo Overdensity (VOD). We combine SEGUE, APOGEE, Gaia, and StarHorse distances to characterize the chemodynamical properties and verify the link between HAC, VOD, and GSE. We find that the orbital eccentricity distributions of the stellar overdensities and GSE are comparable. We also find that they have similar, strongly peaked, metallicity distribution functions, reinforcing the hypothesis of common origin. Furthermore, we show that HAC and VOD are indistinguishable from the prototypical GSE population within all chemical-abundance spaces analyzed. All these evidences combined provide a clear demonstration that the GSE merger is the main progenitor of the stellar populations found within these halo overdensities.