The Milky Way's stellar halo contains debris from dwarf galaxies accreted early in the Galaxy's history, including a population on retrograde (counter-rotating), low-inclination orbits. Kim, Lee & Beers (2026) identified nine substructures in this retrograde component through their metallicity distribution functions in apogalactic-distance–orbital-phase space and interpreted them as debris from at least five distinct progenitors, but the mapping between such chemically defined groups and unique progenitor galaxies remains unverified. The status of Thamnos (one structure, two structures, or largely in-situ contamination), the reality of the newly proposed candidate substructures, and the multi-progenitor interpretation of the retrograde halo are all actively contested in 2024–2026 literature. The open problem is to establish a verified, contamination-quantified mapping between the observed retrograde-halo substructures and the distinct accretion events that produced them, robust to identification-method choices.

Background

In near-field cosmology, the Milky Way's stellar halo is read as a fossil record of the Galaxy's assembly: small dwarf galaxies accreted by the young Milky Way were tidally shredded, and their stars now survive as kinematically coherent debris. Because the quantities conserved during orbital motion — the orbital energy and angular momentum, together called integrals of motion (IoM) — stay roughly clumped for debris from a common progenitor, overdensities in IoM space are used to identify these events. The dominant known event is Gaia-Sausage-Enceladus (GSE), a massive progenitor on a radial, eccentric orbit; retrograde substructures — whose stars orbit opposite to the Galactic disk's rotation — record additional, less understood accretions. Sequoia is a prominent retrograde overdensity at relatively high orbital energy; Thamnos is a low-energy retrograde substructure that Koppelman et al. (2019) split into two components (Thamnos 1 and 2) mainly by stellar metallicity. A metallicity distribution function (MDF) is the histogram of stellar iron abundances, [Fe/H], a proxy for the star-formation history and mass of the birth galaxy; 'chemical tagging' then uses detailed element-abundance patterns to assign stars to common birth environments. The central difficulty is degeneracy: debris from different progenitors overlaps in IoM space, a single merger can produce multiple clumps, in-situ Milky Way stars kinematically heated onto halo-like (including retrograde) orbits contaminate every selection, and the redshift evolution of the galaxy mass–metallicity relation lets galaxies of different masses reach the same [Fe/H] at different epochs, so identical MDF peaks do not by themselves identify a shared progenitor. Building on spectroscopy from the Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), combined with astrometry from the third data release (DR3) of the European Space Agency's Gaia mission, Kim, Lee & Beers (2026) selected low-inclination retrograde stars with eccentricities 0≤e≤0.50 \le e \le 0.5 and identified substructures (labelled LRS 1–5 in two eccentricity bins) from MDF peaks in apogalactic-distance–orbital-phase space, finding that similar MDF peaks can correspond either to one coherent structure or to dynamically distinct groups. They proposed a progenitor classification in which a 'Primary Retrograde Progenitor' (a system undergoing hierarchical tidal stripping with core bifurcation, [Fe/H] ≈−2.3\approx -2.3 to −1.9-1.9) dominates, with a metal-poor progenitor, an early-accreted and a late-accreted progenitor sharing [Fe/H] ≈−1.7\approx -1.7 but separated in orbital energy, and Thamnos 2 accounting for the [Fe/H] ≈−1.5\approx -1.5 component. Orbital parameters depend on the assumed Galactic gravitational potential, and the authors report robustness tests under the Stäckel and McMillan potential models. The authors themselves flag three unresolved issues: MDF-based identification is in principle sensitive to binning choices and metallicity uncertainties; recent studies (Dodd et al. 2023, 2025) suggest Thamnos may be a single dynamical structure rather than the metallicity-based split; and six candidate substructures they identify have no clear association with previously known systems, so the mapping from chemically defined groups to unique progenitor galaxies is incomplete.

Problem Statement

Establish a verified mapping between the chemically and dynamically identified substructures of the Milky Way's low-inclination retrograde stellar halo and the distinct accreted progenitor galaxies that produced them. Concretely, determine: (1) whether Thamnos is a single accreted structure, two structures as in the metallicity-based split of Koppelman et al. 2019, or largely a superposition of in-situ halo, GSE, and other contamination — and, if accreted, its relation to candidate progenitors such as Sequoia, ω Centauri, or the Cetus-Palca stream; (2) whether the candidate substructures newly identified by Kim, Lee & Beers 2026 that lack associations with known systems correspond to real, distinct accretion events, and whether their proposed multi-progenitor classification (a dominant Primary Retrograde Progenitor exhibiting hierarchical tidal stripping with core bifurcation, a metal-poor retrograde progenitor, and early- and late-accreted progenitors sharing [Fe/H] ≈−1.7\approx -1.7 but separated in orbital energy) is correct, or whether these components merge, subdivide, or dissolve into contamination under improved modeling. The mapping must be established with contamination from in-situ and known accreted populations quantified, and must be demonstrated robust to MDF binning choices and metallicity uncertainties (the identification caveats flagged by the source) and to the choice of Galactic gravitational potential (orbital parameters depend on the assumed potential), so that the association between the identified retrograde-halo substructures (including the six candidate substructures without clear associations to previously known systems) and unique progenitor galaxies — incomplete in the source — is established, yielding the overall verdict on whether the retrograde halo is dominated by a single progenitor or is a superposition of multiple accretion events. Determinations (1) and (2), together with the contamination-quantification and robustness conditions above, are jointly required, and none is independently sufficient: resolving the Thamnos status alone, or the reality of the newly identified candidate substructures alone, does not by itself resolve the question. Determine, for every identified retrograde-halo substructure, whether it belongs to a distinct progenitor or is contamination.

References

  1. Kim, Young Kwang; Lee, Young Sun; Beers, Timothy C. 2026, 'Substructures of the Milky Way's Retrograde Halo: Evidence for Multiple Accretion Events', arXiv:2604.13482, https://arxiv.org/abs/2604.13482
  2. Koppelman, Helmer H.; Helmi, Amina; Massari, Davide; Price-Whelan, Adrian M.; Starkenburg, Tjitske K. 2019, 'Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history', Astronomy & Astrophysics 631, L9, doi:10.1051/0004-6361/201936738, https://doi.org/10.1051/0004-6361/201936738
  3. Dodd, Emma; Callingham, Thomas M.; Helmi, Amina; Matsuno, Tadafumi; Ruiz-Lara, Tomás; Balbinot, Eduardo; Lövdal, Sofie 2023, 'Gaia DR3 view of dynamical substructure in the stellar halo near the Sun', Astronomy & Astrophysics 670, L2, doi:10.1051/0004-6361/202244546, https://doi.org/10.1051/0004-6361/202244546 (arXiv:2206.11248)
  4. Dodd, Emma; Ruiz-Lara, Tomás; Helmi, Amina; Gallart, Carme; Callingham, Thomas M.; Cassisi, Santi; Fernández-Alvar, Emma; Surot, Francisco 2025, 'Characterisation of local halo building blocks: Thamnos and Sequoia', Astronomy & Astrophysics 698, A277, doi:10.1051/0004-6361/202451978, https://doi.org/10.1051/0004-6361/202451978 (arXiv:2408.13763)
  5. Ceccarelli, E.; Massari, D.; Palla, M.; Mucciarelli, A.; Bellazzini, M.; Helmi, A. 2025, 'A Walk on the Retrograde Side (WRS) project. II. Chemistry to disentangle in situ and accreted components in Thamnos', arXiv:2510.06332, https://arxiv.org/abs/2510.06332
  6. Xie, Renjing; Yuan, Zhen; Li, Haining; Matsuno, Tadafumi; Martin, Nicolas F.; Zhang, Ruizhi; Yan, Zhiqiang; Sestito, Federico; Thomas, Guillaume F.; Banerjee, Projjwal; Jiang, Ruizheng; Lombardo, Linda; Aguado, David S.; Hattori, Kohei; Zhao, Gang 2026, 'HR-GO II: chemical abundances of low-E retrograde dynamically-tagged-groups: Revealing Thamnos as a very metal-poor substructure', The Astrophysical Journal 999, 69, doi:10.3847/1538-4357/ae39c8, https://doi.org/10.3847/1538-4357/ae39c8 (arXiv:2601.09796)
  7. Kushniruk, Iryna; Youakim, Kristopher; Lind, Karin; Buder, Sven; Kos, Janes; Feuillet, Diane; Martell, Sarah L.; de Grijs, Richard; Lewis, Geraint F.; Bland-Hawthorn, Joss; Da Costa, Gary; Hayden, Michael; Zucker, Daniel; Zwitter, Tomaz; Sharma, Sanjib 2026, 'The GALAH survey: Searching for and characterizing halo substructures with the GALAH DR4 survey', Astronomy & Astrophysics 709, A69, doi:10.1051/0004-6361/202451201, https://doi.org/10.1051/0004-6361/202451201 (arXiv:2602.19647)
  8. Mori, A.; Di Matteo, P.; Salvadori, S.; Mondelin, M.; Khoperskov, S.; Haywood, M.; Mastrobuono-Battisti, A. 2025, 'Chemical decoding of kinematic substructures in the Galactic halo', arXiv:2509.13408, https://arxiv.org/abs/2509.13408
  9. Molaro, Paolo; Bonifacio, Piercarlo; Caffau, Elisabetta; Monaco, Lorenzo; Cescutti, Gabriele 2026, 'Beryllium enhancement in stars of the accreted Thamnos-2 system', arXiv:2607.08179, https://arxiv.org/abs/2607.08179
  10. Sante, Andrea; Font, Andreea S.; Kawata, Daisuke; Makinen, T. Lucas; Grand, Robert J. J. 2026, 'A simulation-based inference of the Milky Way merger history', arXiv:2603.12317, https://arxiv.org/abs/2603.12317
  11. Naidu, Rohan P.; Conroy, Charlie; Bonaca, Ana; Johnson, Benjamin D.; Ting, Yuan-Sen; Caldwell, Nelson; Zaritsky, Dennis; Cargile, Phillip A. 2020, 'Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure', The Astrophysical Journal 901, 48, doi:10.3847/1538-4357/abaef4, https://doi.org/10.3847/1538-4357/abaef4 (arXiv:2006.08625)