Chemical decoding of kinematic substructures in the Galactic halo

Abstract: In the hierarchical mass assembly framework, the accretion history of the Milky Way is crucial to understand its evolution. Previous studies have shown that integrals of motion (such as energy and angular momentum) are not strictly conserved during massive mergers, leading to a broad redistribution of accreted stars across dynamical spaces. Additionally, part of the in-situ disc component itself becomes kinematically heated and acquires halo-like orbits as a result of the merger. Consequently, even for minor mergers, which experience weaker dynamical friction and are then supposed to preserve a higher degree of dynamical coherence, we expect their kinematic-defined samples to be anyway contaminated by both the massive merger(s) and the disc stars, thereby hiding the presence of different populations within their samples.

This study aims at quantifying these contamination effects in known accreted halo substructures. As they are defined by their kinematics, we aim at cleaning their samples analysing their chemical properties only, to uncover their specific abundance patterns.
We applied the kinematic selection criteria for the halo substructures to the Gaia EDR3 and APOGEE DR17 data. Then we adopted a Gaussian Mixture Model (GMM) approach to chemically compare different substructures, taking into account several abundances (Fe, Mg, Si, Ca, Mn, Al, and C). This method incorporates the analysis of various elements that probe different nucleosynthetic channels, providing a percentage of global chemical compatibility. At the same time, it allows us to perform a comprehensive comparison between two samples of the overall distribution of a given element [X/Fe] vs. [Fe/H]. 

We show that a number of halo substructures feature a high fraction of chemically-compatible stars with respect to the most massive merger of the MW (i.e., GSE), along with some of them also displaying a non-negligible contamination from the disc. After removing contamination from both components, we derive the intrinsic chemical patterns of the known accreted halo substructures. We argue that the chemical properties of some halo substructures point towards a shared origin with GSE or at least to a very similar chemical evolution history.