Chemical decoding of kinematic substructures in the Galactic halo

Abstract

In the hierarchical assembly framework, the accretion history of the Milky Way is crucial to understand its evolution. However, in massive mergers, integrals of motion are not strictly conserved, redistributing accreted stars across dynamical spaces, such as energy-angular momentum ($E-L_z$). Additionally, the in situ disc becomes kinematically heated, acquiring halo-like orbits. Consequently, even for minor mergers, which should preserve dynamical coherence, we expect their kinematic-defined samples to be contaminated by both the massive merger(s) and the disc stars. This study aims at quantifying this contamination in known accreted halo substructures. As they are defined by kinematics, we aim at cleaning their samples analysing only chemical properties. 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 approach to chemically compare different substructures on a star-by-star basis, taking into account several abundances (Fe, Mg, Si, Ca, Mn, Al, and C). We argue that the chemical properties of Sequoia point towards a shared origin with GSE. Heracles, Thamnos and the Helmi Stream all likely comprise GSE and heated disc stars in a significant amount. Besides these two populations, we identified stars with chemical and orbital properties compatible with Sagittarius in the Helmi Stream and with $\omega$ Cen in Thamnos. Finally, GSE itself is contaminated by Sagittarius. Halo stars chemically compatible with GSE are spread throughout the $E-L_z$ space and considerably contaminate every halo substructure studied in this work. None of these substructures appears to be a unique population of stars with its own origin. In addition to GSE, they all appear to be mixtures of stars chemically compatible either with the metal-poor disc, Sagittarius, $\omega$ Cen, or with a combination of them.