Origins of the new inner-halo substructure candidates from APOGEE DR17 chemo-dynamical clustering (NSTC-1/2/3, HAS/Nyx–Nyx-2, LAS, UDG) and the multiplicity of the GSE accretion

Unsupervised clustering of APOGEE DR17 inner-halo stars in a seven-dimensional chemo-dynamical space (Li, Du, Liu & Ye 2025) recovered the known Gaia-Sausage-Enceladus, Sagittarius, LMS-1/Wukong, Thamnos, and metal-weak thick disk substructures, and reported six new or ambiguously attributed groups: three new-substructure candidates (NSTC-1/2/3) with dwarf-galaxy-like low [Mg/Fe] and [Al/Fe] but disk-like dynamics, a high-alpha structure (HAS) identified with the Nyx and Nyx-2 streams, a low-alpha group (LAS) attributed to the thin disk, and an undefined metal-poor group (UDG) tentatively linked to Aleph. The paper itself states that progenitor assignments from such clustering remain tentative and that the candidates require independent validation. As of August 2026, none of these candidates has been independently confirmed, refuted, or assigned a progenitor, and the question of whether the metal-rich Gaia-Sausage-Enceladus component implies an extended or multiple accretion history rather than Splash/heated-disk contamination remains open.

Background

The Milky Way's stellar halo is built largely by hierarchical accretion: dwarf galaxies disrupted in the past leave their stars as diffuse debris whose orbital properties and chemical composition still cluster together. Because orbital energy $E$ and angular momentum are approximately conserved, and because stars retain the element-abundance pattern of their birth environment, accreted satellites remain detectable as clumps ('substructures') in integrals-of-motion and abundance spaces long after their spatial streams are erased. Key quantities: [Fe/H] (iron abundance, a metallicity tracer), [ $\alpha$ /Fe] traced by [Mg/Fe] (slowly enriched by core-collapse supernovae, so high values indicate rapid star formation), [Al/Fe], and the actions $L_z$ , $J_r$ , $J_z$ (the conserved vertical angular momentum and radial/vertical oscillation amplitudes of a stellar orbit) together with orbital energy $E$ . The dominant known inner-halo substructure is Gaia-Sausage-Enceladus (GSE), the debris of the Milky Way's last major merger, on radial orbits; the 'Splash' is a population of in-situ disk stars dynamically heated onto halo-like orbits by that same merger, which overlaps GSE kinematically. Other recognized structures include the Sagittarius stream, LMS-1/Wukong, Thamnos, the metal-weak thick disk (MWTD), Aleph (a compact, metal-rich overdensity in the inner Galaxy), and the prograde Nyx stream (a stellar stream moving in the direction of Galactic rotation; Necib et al. 2020), with a counterpart 'Nyx-2' having opposite radial velocities. Li, Du, Liu & Ye (2025, ApJ 985, 47) applied StarGO — an unsupervised clustering pipeline (a t-SNE projection — t-distributed stochastic neighbor embedding, a nonlinear dimensionality-reduction method — followed by a self-organizing map, a neural network that maps high-dimensional data onto a trainable low-dimensional grid of cells) developed to find stellar groups in high-dimensional data — to APOGEE DR17 (the final data release of the Apache Point Observatory Galactic Evolution Experiment high-resolution near-infrared spectroscopic survey) in the 7-D space ([Fe/H], [Mg/Fe], [Al/Fe], $L_z$ , $J_r$ , $J_z$ , $E$ ). In a first pass restricted to metal-poor stars ([Fe/H] < -0.8) they identified 42 groups assigned to GSE, LMS-1/Wukong, Thamnos, Sagittarius, and MWTD, together with additional candidate groups including an undefined, metal-poor group (UDG — the acronym means 'undefined group', not 'ultra-diffuse galaxy') tentatively linked to Aleph; in a second pass without the metallicity pre-cut they recovered metal-rich GSE members, confirmed the presence of Aleph, and found NSTC-1, NSTC-2, NSTC-3 (new-substructure candidates 1, 2, and 3; 37, 32, and 40 member stars; NSTC-2 and NSTC-3 at >5 sigma significance), HAS (a high-alpha substructure), and LAS (a low-alpha structure). The NSTCs have disk-like kinematics ( $L_z \sim [1.2, 3.8]\times 10^3$ kpc km/s, low $J_r$ , $J_z$ ) but unusually low [Mg/Fe] (< 0.2 dex) and [Al/Fe] (< -0.15 dex) — dwarf-galaxy-like chemistry — at relatively high metallicity ([Fe/H] > -1.15) and orbital energy, which the authors interpret as possible recent ex-situ accretion. HAS members split into stars matching Nyx and stars matching Nyx-2, and their chemical coherence is offered as support for a common origin of the two streams. Three caveats frame the open question. First, distinct clumps in action space need not be distinct satellites: dynamical friction (orbital decay of a massive satellite, which breaks $E$ – $L_z$ conservation) can spread one progenitor's debris over wide regions of kinematic space. Second, the Galactic bar can resonantly trap stars so that orbital-resonance overdensities mimic merger relics. Third, clustering results depend on the SOM hyperparameters and on the metallicity pre-selection, so the completeness of the identified groups is unquantified and their contamination is only model-estimated. The authors therefore call for validation with future astrometric, photometric, and spectroscopic data — Gaia DR4 (the next data release of the Gaia space-astrometry mission), CSST/LSST wide-field survey photometry (Chinese Space Station Telescope; the Legacy Survey of Space and Time at the Vera C. Rubin Observatory), and WEAVE/4MOST multi-object spectroscopy (next-generation ground-based spectroscopic surveys) — and with high-resolution cosmological zoom-in simulations that track stellar abundances.

Problem Statement

Determine the physical reality and origin of the substructure candidates that Li et al. (2025) detected by StarGO clustering of APOGEE DR17 stars in the 7-D chemo-dynamical space ([Fe/H], [Mg/Fe], [Al/Fe], $L_z$ , $J_r$ , $J_z$ , $E$ ), and settle the accompanying question about the metal-rich Gaia-Sausage-Enceladus component. Specifically: (1) Are NSTC-1, NSTC-2, and NSTC-3 genuine, distinct stellar populations rather than artifacts of the clustering thresholds or of the metallicity pre-selection — and is their suggested recent ex-situ (accreted satellite) origin, inferred from low [Mg/Fe], low [Al/Fe], disk-like dynamics, and high orbital energy, correct, and if so what progenitor(s) produced them? (2) Is HAS truly the combined debris of the Nyx and Nyx-2 streams with a common origin as its chemical coherence is claimed to support, or are HAS/Nyx/Nyx-2 instead in-situ disk or thick-disk stars heated or radially migrated by a merger, or overdensities induced by bar-resonant trapping? Is LAS merely part of the Galactic thin disk, and is UDG physically linked to Aleph? (3) Are the metal-rich GSE members genuine GSE debris implying accretion that continued after thin-disk formation and possibly multiple accretion events, or are they Splash stars or otherwise heated in-situ disk contaminating the GSE selection? The physical reality and origin of any one of the named candidate structures — NSTC-1/2/3, HAS/Nyx–Nyx-2, LAS, UDG — or of the metal-rich GSE component is an independent question. Answers must rest on evidence independent of the original clustering and must address whether distinct kinematic clumps imply distinct progenitors given dynamical friction and resonant trapping, together with the completeness and contamination of the original group identification.

References

Li, Zhongcheng; Du, Cuihua; Liu, Haoyang; Ye, Dashuang (2025). 'Investigation of Substructures of the Inner Halo Using the StarGO Algorithm with APOGEE DR17', The Astrophysical Journal 985, 47. DOI: 10.3847/1538-4357/adc809, https://doi.org/10.3847/1538-4357/adc809 (preprint arXiv:2504.01536, https://arxiv.org/abs/2504.01536, posted under the slightly different preprint title 'Investigate Substructures of the Inner Halo Using the StarGO Algorithm with APOGEE DR17')
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