Unsupervised Chemo-Dynamical Dissection of the Inner Galactic Halo: Discovery of Five Accreted Substructures with SDSS-V and Gaia

Akbaba F., Plevne O.

PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, cilt.138, sa.7, 2026 (SCI-Expanded, Scopus)

Özet

The inner Galactic halo is a complex graveyard of the Milky Way's earliest accretion events, where severe orbital phase-mixing challenges traditional dynamical stream-finding techniques. In this work, we present a purely data-driven, 12-dimensional chemo-dynamical analysis of the inner halo using the latest data from the SDSS-V Milky Way Mapper (DR19) and Gaia DR3. Utilizing an unsupervised machine learning framework based on UMAP and HDBSCAN, we perform a blind search for clustered populations within a chemically selected ex-situ sample of 2185 stars without imposing kinematic pre-selection criteria. Our pipeline successfully recovers nine kinematic groupings corresponding to seven previously known canonical substructures (including Gaia-Enceladus/Sausage, the Helmi Streams, and Sequoia), validating the robustness of the high-dimensional feature space. Furthermore, we report the discovery of five new tightly bound candidate substructures, designated FO1-FO5 (Etot <= -1.8 & times; 105 km2 s-2). Statistical validation confirms four of the five candidates (FO1, FO3, FO4, and FO5) as robust chemo-dynamical overdensities, while the tentative candidate FO2 exhibits a striking nitrogen enhancement ([N/Fe] = +0.83 +/- 0.16) indicative of the tidal debris from a disrupted massive globular cluster. Finally, we demonstrate that high-dimensional chemical information is critical for resolving dynamical degeneracies in the crowded inner halo, clearly differentiating structures that share similar orbits but distinct chemical signatures (e.g., FO5 and Shiva), as well as the reverse case where chemical similarity masks dynamical differences (e.g., FO3 and the Helmi Streams). These findings emphasize that the deepest regions of the Galactic potential remain far from fully mapped, preserving a rich and diverse record of the Galaxy's assembly history.