The two faces of Gaia-Sausage-Enceladus

Mining the chemical abundance space with graph attention networks

¹ INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
² Dipartimento di Fisica e Astronomia, Università di Padova, 35122 Padova, Italy
³ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ INAF - IASF-Milano, Via Alfonso Corti 12, 20133 Milano, Italy
⁵ University of Birmingham, School of Computer Science, Birmingham B15 2TT, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 November 2025
Accepted: 29 January 2026

Abstract

Aims. Recent studies suggest that chemical abundances hold the key to disentangling halo substructures, providing a more reliable tracer than dynamics alone. We aim to probe the Milky Way stellar halo using high-dimensional chemical abundances from GALAH DR4. By leveraging multiple nucleosynthesis channels in synergy with integrals of motion (IoM), we extract information hidden in the raw abundance space to perform chemical tagging.

Methods. With a graph attention autoencoder, we reconstructed a dynamics-informed, denoised chemical space and identified coherent stellar substructures by applying ensemble clustering.

Results. Our method successfully recovers the three largest globular clusters hidden in the dataset, estimates the in situ fraction to be approximately 41%, and chemically characterizes several dynamical halo substructures. Strikingly, stars dynamically associated with Gaia-Sausage-Enceladus (GSE) separate into two chemically distinct clusters. By examining their abundances, energy (E), and angular momentum (L_z) distributions, together with the metallicity trend with E, we connect these clusters to their birthplace within the progenitor by proposing a simple infall scenario: one cluster traces the metal-poor, less evolved outskirts, while the other traces the metal-rich, chemically evolved core.

Conclusions. These findings support the picture of GSE as a massive disk galaxy with a noticeable negative metallicity gradient. Moreover, selecting potential GSE members based on their chemical properties rather than solely on their dynamics naturally expands the membership to stars spanning a wider range of IoM.