Cosmological Simulations of Stellar Halos with Gaia Sausage-Enceladus Analogues: Two Sausages, One Bun?

TL;DR

This study uses cosmological simulations to explore whether the Gaia Sausage--Enceladus stellar debris in the Milky Way's halo originates from one or multiple merger events, highlighting differences in chemical and age properties.

Contribution

It identifies the presence of single- and two-merger Gaia Sausage--Enceladus analogues in simulations and emphasizes the importance of chemical and age data in distinguishing their origins.

Findings

Two-merger GSEs account for a third of cases.

Single-merger GSEs are accreted more recently than two-merger GSEs.

Chemical and age differences help distinguish merger scenarios.

Abstract

Observations of the Milky Way's stellar halo find that it is predominantly comprised of a radially biased population of stars, dubbed the Gaia Sausage--Enceladus, or GSE. These stars are thought to be debris from dwarf galaxy accretion early in the Milky Way's history. Though typically considered to be from a single merger, it is possible that the GSE debris has multiple sources. To investigate this possibility, we use the TNG50 simulation to identify stellar accretion histories in 98 Milky Way analogues -- the largest sample for which such an identification has been performed -- and find GSE-like debris in 32, with two-merger GSEs accounting for a third of these cases. Distinguishing single-merger GSEs from two-merger GSEs is difficult in common kinematic spaces, but differences are more evident through chemical abundances and star formation histories. This is because single-merger GSEs are typically accreted more recently than the galaxies in two-merger GSEs: the median infall times (with 16th and 84th percentiles) are $5.9^{+3.3}_{-2.0}$ and $10.7^{+1.2}_{-3.7}$ Gyr ago for single- and two-merger scenarios, respectively. The systematic shifts in abundances and ages that occur as a result suggest that efforts in modeling these aspects of the stellar halo prove ever-important in understanding its assembly.