Two faces of Gaia-Sausage-Enceladus: Mining the chemical abundance space with graph attention networks

Abstract

Recent studies suggest that chemical abundances hold the key to disentangling halo substructure, 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. With a graph attention autoencoder, we reconstruct a dynamics-informed, denoised chemical space and identify coherent stellar substructures by applying ensemble clustering. 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.

arXiv comment18 pages, 11 figures. Accepted for publication in A&A

References in corpus (67)

• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• The New Galaxy: Signatures of its Formation
• Tracing Galaxy Formation with Stellar Halos I: Methods
• Co-formation of the Galactic disc and the stellar halo
• The GALAH Survey: Scientific Motivation
• Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way
• Presupernova evolution and explosive nucleosynthesis of rotating massive stars in the metallicity range -3 <=[Fe/H]<= 0
• Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
• Quantifying the uncertainties of chemical evolution studies. II. Stellar yields
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• Evolution, nucleosynthesis and yields of AGB stars at different metallicities (III): intermediate mass models, revised low mass models and the ph-FRUITY interface
• The Biggest Splash
• The origin of accreted stellar halo populations in the Milky Way using APOGEE, $\text{Gaia}$, and the EAGLE simulations
• The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation of spiral galaxies
• The birth of the Milky Way as uncovered by accurate stellar ages with Gaia
• Shredded Galaxies as the Source of Diffuse Intrahalo Light On Varying Scales
• Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy
• The Milky Way has no in-situ halo other than the heated thick disc. Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE
• Using chemical tagging to redefine the interface of the Galactic disk and halo
• The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation
• Gaia reveals a metal-rich in-situ component of the local stellar halo
• Atomic data for the Gaia-ESO Survey
• Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba
• Assembly History and Structure of Galactic Cold Dark Matter Halos
• Mapping the Stellar Halo with the H3 Spectroscopic Survey
• Overview of the DESI Milky Way Survey
• Ages and kinematics of chemically selected, accreted Milky Way halo stars
• Formation of In Situ Stellar Haloes in Milky Way-Mass Galaxies
• The chemical characterisation of halo substructure in the Milky Way based on APOGEE
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• Stellar haloes of simulated Milky Way-like galaxies: Chemical and kinematic properties
• The origin and evolution of the odd-Z iron-peak elements Sc, V, Mn, and Co in the Milky Way stellar disk
• The GALAH survey: An abundance, age, and kinematic inventory of the solar neighbourhood made with TGAS
• On the kinematic detection of accreted streams in the Gaia era: a cautionary tale
• The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars
• The evolution of CNO isotopes: the impact of massive stellar rotators
• The GALAH Survey: Chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and $Gaia$ eDR3
• Elevated r-process enrichment in Gaia Sausage and Sequoia
• The Gaia DR3 view of dynamical substructure in the stellar halo near the Sun
• GASTRO library I: the simulated chemodynamical properties of several GSE-like stellar halos
• The Gaia-ESO Survey: a quiescent Milky Way with no significant dark/stellar accreted disc
• Evidence from Disrupted Halo Dwarfs that $r$-process Enrichment via Neutron Star Mergers is Delayed by $\gtrsim 500$ Myrs
• Constraints of the physics of low-mass AGB stars from CH and CEMP stars
• The R-Process Alliance: Chemo-Dynamically Tagged Groups of Halo $r$-Process-Enhanced Stars Reveal a Shared Chemical-Evolution History
• Substructure in the stellar halo near the Sun. II. Characterisation of independent structures
• Substructure in the stellar halo near the Sun. I. Data-driven clustering in Integrals of Motion space
• Mapping Substructures in Dark Matter Halos
• The detailed chemical abundance patterns of accreted halo stars from the optical to infrared
• A Comparative Analysis of the Chemical Compositions of Gaia-Enceladus/Sausage and Milky Way Satellites using APOGEE
• The (im)possibility of strong chemical tagging
• Non-LTE analysis of copper abundances for the two distinct halo populations in the solar neighborhood
• The chemical evolution of the dwarf Spheroidal galaxy Sextans
• Metallicity distributions of halo stars: do they trace the Galactic accretion history?
• The stellar halo in Local Group Hestia simulations I. The in-situ component and the effect of mergers
• A Swing of the Pendulum: The Chemodynamics of the Local Stellar Halo Indicate Contributions from Several Radial Merger Events
• Copper abundance from Cu I and Cu II lines in metal-poor star spectra: NLTE vs LTE
• The stellar halo in Local Group Hestia simulations II. The accreted component
• Photometric metallicity for 694233 Galactic giant stars from Gaia DR3 synthetic Stromgren photometry. Metallicity distribution functions of halo sub-structures
• Detection of Al II in the Ultraviolet Spectra of Metal-Poor Stars: An Empirical LTE Test of NLTE Aluminum Abundance Calculations
• The Gaia-ESO Survey: Chemical tagging in the thin disk. Open clusters blindly recovered in the elemental abundance space
• Revisiting inelastic Cu+H collisions and the non-LTE Galactic evolution of copper
• Evidence of Gaia Enceladus experiencing at least two passages around the Milky Way
• How well can we unravel the accreted constituents of the Milky Way stellar halo? A test on cosmological hydrodynamical simulations
• Shaping Galactic Habitability: the impact of stellar migration and gas giants
• From order to chaos: the blurred out metallicity gradient of the Gaia-Enceladus/Sausage progenitor
• The origin of elements: the need for UV spectra