Investigation of Substructures of the Inner Halo Using the StarGO Algorithm with APOGEE DR17

Abstract We applied the Stars’ Galactic Origin algorithm in 7D space (i.e., [Fe/H], [Mg/Fe], [Al/Fe], L z , J r , J z , E ) to analyze stars in the inner halo with APOGEE DR17. We identified some known substructures, including Gaia-Sausage/Enceladus, Sagittarius stream (Sgr), LMS-1 (Wukong), Thamnos, the metal-weak thick disk, and Aleph. Additionally, we identified an undefined metal-poor group (UDG, with [Fe/H] < −0.8 dex) probably linked to a known substructure like Aleph, as well as a high α -abundance substructure (HAS) associated with both the Nyx and Nyx-2 streams. Chemical abundance of the HAS supports the argument that Nyx and Nyx-2 share a common origin. We discovered three substructures, which we refer to as new substructure candidates 1, 2, and 3 (NSTC-1, NSTC-2, NSTC-3). Despite exhibiting disk-like dynamics, these NSTCs demonstrate notably low [Mg/Fe] (<0.2 dex) and [Al/Fe] (<−0.15 dex), similar to the properties of dwarf galaxies. Their high orbital energy and low [ α /Fe] indicate the association with recent accretion events.