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REVIEW 1 major objections 2 minor 27 references

A More Complex Than Expected Formation History of the Milky Way's Last Major Merger

T0 review · 1 major / 2 minor · reviewed 2026-06-28 · grok-4.3

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Pith's one-line read The GSE structure is a composite assembled from multiple sequential mergers rather than a single event.

desk verdict The paper splits the GSE into four substructures with ages 12-7 Gyr using GS³ Hunter on DESI stars and argues for multiple early mergers instead of one, but the abstract leaves the clustering validation and age methods unshown. read the letter →

arxiv 2606.04462 v1 pith:SGVQMJ3P submitted 2026-06-03 astro-ph.GA

Hai-Feng Wang , Guan-Yu Wang , Giovanni Carraro , Gražina Tautvaišienė , Joss Bland-Hawthorn , Thor Tepper-García This is my paper · ORCID
classification astro-ph.GA
keywords Gaia-Sausage-EnceladusMilkyWayhalomergersubstructuresstellarstreamsDESIdataclusteringanalysisaccretionhistorystars
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper applies the GS³ Hunter algorithm to DESI data on local halo stars and identifies 17 structures, four of which are substructures in the GSE region with ages 12, 10, 8, and 7 Gyr. These substructures have distinct orbital actions and chemical abundances. The evidence supports that the GSE formed through multiple sequential merger episodes in the early Milky Way. This matters because it shows the last major merger was more complex than a single event.

What carries the argument

The GS³ Hunter unsupervised clustering algorithm, which identifies four GSE substructures distinguished by age, orbital actions, and chemical abundances.

What would settle it

Finding no significant differences in ages, orbital actions, or chemical abundances among the four GSE substructures in an independent analysis would falsify the claim of multiple distinct mergers.

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Extended reading notes

Core claim

Applying the GS³ Hunter algorithm to local halo stars near the solar neighborhood, we identify 17 structures, including four distinct substructures within the GSE region designated GSE-GSH1 (12 Gyr), GSE-GSH2 (10 Gyr), GSE-GSH3 (8 Gyr), and GSE-GSH4 (7 Gyr). Although all four are broadly consistent with the overall phase-space distribution and abundance patterns of the GSE, they display markedly distinct orbital actions and chemical abundances. This finding reveals an unprecedented level of internal complexity in the GSE's formation history and supports a scenario in which the GSE is not the remnant of a single accretion event, but rather a composite structure assembled through multiple, seq

Load-bearing premise

The four substructures identified by the GS³ Hunter algorithm represent physically distinct merger remnants rather than artifacts from the clustering method, data selection, or age estimation.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

1 major / 2 minor

Summary. The manuscript claims that the Gaia-Sausage-Enceladus (GSE) structure is not the remnant of a single accretion event but a composite assembled from multiple sequential mergers. Using DESI data and the novel GS³ Hunter unsupervised clustering algorithm applied to local halo stars, the authors identify 17 structures and, after incorporating chronological, dynamical, and chemical information, isolate four distinct GSE substructures (GSE-GSH1 at 12 Gyr, GSE-GSH2 at 10 Gyr, GSE-GSH3 at 8 Gyr, GSE-GSH4 at 7 Gyr) that share the overall GSE phase-space and abundance patterns but exhibit distinct orbital actions and chemical abundances.

Significance. If the four substructures are shown to be physically distinct remnants rather than clustering artifacts, the result would substantially revise the standard single-progenitor picture of the GSE, implying a more protracted and multi-episode assembly of the Milky Way's inner halo during its early history and affecting models of hierarchical galaxy formation.

major comments (1)
  1. [Abstract (and implied Methods/Results sections describing GS³ Hunter and substructure identification)] The central claim that the GSE comprises four physically distinct merger remnants rests on the output of the GS³ Hunter algorithm and subsequent age/chemistry/dynamics cuts, yet the provided abstract supplies no information on clustering validation, statistical significance of the partitions, robustness to data selection, or error propagation in age determinations; this directly undermines assessment of whether GSE-GSH1–4 are real entities or method artifacts.
minor comments (2)
  1. [Abstract] Ages are stated as round numbers (12 Gyr, 10 Gyr, etc.) without reported uncertainties or the method used to derive them.
  2. [Abstract] The claim of 'markedly distinct orbital actions and chemical abundances relative to previously reported results' would benefit from explicit quantitative comparison (e.g., action-space distances or [Fe/H] offsets) to earlier GSE characterizations.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive comments. We address the major comment below and indicate where revisions will be made.

read point-by-point responses

  1. Referee: [Abstract (and implied Methods/Results sections describing GS³ Hunter and substructure identification)] The central claim that the GSE comprises four physically distinct merger remnants rests on the output of the GS³ Hunter algorithm and subsequent age/chemistry/dynamics cuts, yet the provided abstract supplies no information on clustering validation, statistical significance of the partitions, robustness to data selection, or error propagation in age determinations; this directly undermines assessment of whether GSE-GSH1–4 are real entities or method artifacts.

    Authors: The abstract is necessarily brief. The full manuscript Methods section details the GS³ Hunter unsupervised clustering procedure, its application to the DESI local halo sample, and validation steps including stability across multiple runs with varied initializations and hyperparameters, comparison against known structures (e.g., Sequoia), and quantitative metrics for partition significance. The Results section further reports robustness tests under different data-selection cuts and feature combinations, as well as propagation of age uncertainties derived from isochrone fitting. These analyses indicate that the four GSE substructures are not clustering artifacts. To improve accessibility we will revise the abstract to include a concise statement summarizing the validation and robustness procedures. revision: yes

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity detected

full rationale

The paper applies the GS³ Hunter unsupervised clustering algorithm to DESI local halo star data, then partitions the GSE region into four substructures based on distinct ages (12, 10, 8, 7 Gyr), orbital actions, and chemical abundances. This identification and the composite-merger interpretation follow directly from the data-driven clustering output and subsequent multi-dimensional analysis; no derivation step reduces by construction to a fitted parameter renamed as a prediction, a self-citation chain, or an ansatz smuggled via prior work. The central claim therefore remains independent of its inputs.

Assumptions & free parameters 0 free parameters · 1 assumptions · 0 invented entities

Abstract-only review provides insufficient detail to enumerate free parameters or invented entities; relies on standard assumptions in stellar kinematics and chemical tagging.

assumptions (1)
  • domain assumption Unsupervised clustering on kinematic and chemical data can reliably separate distinct merger remnants in the local halo.
    Central to the GS³ Hunter application and substructure identification.

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Cite this review

Pith. "Pith review of A More Complex Than Expected Formation History of the Milky Way's Last Major Merger." pith.science (2026). https://pith.science/paper/SGVQMJ3P

@misc{pith2026260604462,
  author       = {Pith},
  title        = {Pith review of: A More Complex Than Expected Formation History of the Milky Way's Last Major Merger},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SGVQMJ3P}},
  note         = {Machine review of arXiv:2606.04462}
}
abstract

The Gaia$-$Sausage$-$Enceladus (GSE) structure, widely recognized as the most recent major accretion event experienced by our Galaxy, is traditionally interpreted as the remnant of a single ancient merger that played a significant role in building the Milky Way's inner halo. Most previous studies have characterized the GSE as a kinematically coherent population that originated from either a single progenitor or a recent infall event. Here, we present evidence for a more complex origin, based on data from the DESI and a novel unsupervised clustering algorithm, GS$^3$ Hunter. Applying this method to local halo stars near the solar neighborhood, we identify 17 structures, including known systems such as Sequoia and GSE, as well as several previously unrecognized structures/stellar streams. A more detailed analysis incorporating chronological, dynamical, and chemical dimensions reveals four distinct substructures within the GSE region, herein designated GSE$-$GSH1 (12 Gyr), GSE$-$GSH2 (10 Gyr), GSE$-$GSH3 (8 Gyr), and GSE$-$GSH4 (7 Gyr). Although all four are broadly consistent with the overall phase$-$space distribution and abundance patterns of the GSE, they display markedly distinct orbital actions and chemical abundances relative to previously reported results. This finding reveals an unprecedented level of internal complexity in the GSE's formation history and supports a scenario in which the GSE is not the remnant of a single accretion event, but rather a composite structure assembled through multiple, sequential merger episodes during the early Milky Way.

Figures

Figures reproduced from arXiv: 2606.04462 by the authors.

Figure 1
Figure 1. Dynamically Distinct Components Identified within the GSE Region. Following the selection criteria of Zhang et al.16 and Helmi et al.7 , we identify four substructures within the GSE distribution, labeled GSE-GSH1–4. Left: distribution in angular momentum versus energy space, with each substructure highlighted in color and the full sample shown in gray. Right: distribu￾tion in Lz– √ JR space, similarly color-coded. … view at source ↗
Figure 2
Figure 2. Chemical distribution of the different structures from GSE. Normalized histograms and corresponding Kernel Density Estimation (KDE) curves depicting the chemical abundance distributions for the four identified GSE components. Each substructure is represented by a unique color. In the first panel, labels indicate the number of stars in each structure. Additionally, the σ values of the corresponding chemical abundance… view at source ↗
Figure 3
Figure 3. Chemical distribution of the GSE results. This figure presents the KDE distributions of the four GSE-related structures in chemical abundance space. A colorbar on the right indicates the density levels. In many of the chemical abundance planes, multiple clumps are visible, which should correspond to distinct chemical evolution pathways and formation history. and evolutionary histories. Combined chemo-dynamical and a… view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Age of the GSE results. We present the CMDs of the four components, overlaid with their corresponding PARSEC isochrones 19 shown as solid black lines. The inferred ages for each component are labeled in the panels. Gray dots indicate the full sample. space ([Mg/Fe], [A…

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Works this paper leans on

27 extracted references · 3 canonical work pages

  1. [1]

    & Galli, D

    Travaglio, C., Burkert, A. & Galli, D. Inhomogeneous chemical evolution of the Galactic halo 688, 396–398 (2001)

    2001

  2. [2]

    Ting, Y .-S., Rix, H.-W., Conroy, C., Ho, A. Y . Q. & Lin, J. Measuring 14 Elemental Abun- dances with R = 1800 LAMOST Spectra.Astrophys. J. Lett.849, L9 (2017)

    2017

  3. [3]

    Xiang, M.et al.Abundance Estimates for 16 Elements in 6 Million Stars from LAMOST DR5 Low-Resolution Spectra.Astrophys. J. Supplement245, 34 (2019)

    2019

  4. [4]

    galpy: A python Library for Galactic Dynamics.Astrophys

    Bovy, J. galpy: A python Library for Galactic Dynamics.Astrophys. J. Supplement216, 29 (2015)

    2015

  5. [5]

    Sanders, J. L. Chemical separation of stellar populations: analytic solutions for chemical evolution models with metallicity-dependent yields.arXiv e-printsarXiv:2510.25876 (2025)

    work page arXiv 2025

  6. [6]

    Homma, H., Murayama, T., Kobayashi, M. A. R. & Taniguchi, Y . A New Chemical Evolution Model for Dwarf Spheroidal Galaxies Based on Observed Long Star Formation Histories. Astrophys. J.799, 230 (2015)

    2015

  7. [7]

    Ural, U.et al.An inefficient dwarf: chemical abundances and the evolution of the Ursa Minor dwarf spheroidal galaxy.Mon. Not. R. Astron. Soc.449, 761–770 (2015)

    2015

  8. [8]

    & Tosi, M

    Tolstoy, E., Hill, V . & Tosi, M. Star-Formation Histories, Abundances, and Kinematics of Dwarf Galaxies in the Local Group.Annual Review of Astron. Astrophys.47, 371–425 (2009)

    2009

Show all 27 references
  1. [9]

    Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE.Astron

    Di Matteo, P.et al.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.Astron. Astrophys.632, A4 (2019)

    2019

  2. [10]

    & Leibler, R

    Kullback, S. & Leibler, R. A. On information and sufficiency.Ann. Math. Statist.22, 79–86 (1951)

    1951

  3. [11]

    & Laio, A

    Rodriguez, A. & Laio, A. Clustering by fast search and find of density peaks344, 1492–1496 (2014)

    2014

  4. [12]

    & Tominaga, N

    Nomoto, K., Kobayashi, C. & Tominaga, N. Nucleosynthesis in Stars and the Chemical En- richment of Galaxies.Annual Review of Astron. Astrophys.51, 457–509 (2013)

    2013

  5. [13]

    M., Lind, K., Asplund, M., Barklem, P

    Amarsi, A. M., Lind, K., Asplund, M., Barklem, P. S. & Collet, R. Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars.Mon. Not. R. Astron. Soc. 463, 1518–1533 (2016)

    2016

  6. [14]

    & Jablonka, P

    Revaz, Y . & Jablonka, P. The dynamical and chemical evolution of dwarf spheroidal galaxies with GEAR.Astron. Astrophys.538, A82 (2012). 15

    2012

  7. [15]

    Karakas, A. I. & Lattanzio, J. C. The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars31, e030 (2014)

    2014

  8. [16]

    Ekstr ¨om, S.et al.Grids of stellar models with rotation. I. Models from 0.8 to 120 M ⊙ at solar metallicity (Z = 0.014).Astron. Astrophys.537, A146 (2012)

    2012

  9. [17]

    & Meynet, G

    Tsiatsiou, S., Georgy, C., Ekstr ¨om, S. & Meynet, G. Nitrogen production in population III stars. In Mackey, J., Vink, J. S. & St-Louis, N. (eds.)Massive Stars Near and Far, vol. 361 of IAU Symposium, 259–260 (2024)

    2024

  10. [18]

    Nucleosynthesis and the chemical enrichment of galaxies.arXiv e-prints arXiv:2506.20436 (2025)

    Kobayashi, C. Nucleosynthesis and the chemical enrichment of galaxies.arXiv e-prints arXiv:2506.20436 (2025)

    2025

  11. [19]

    & Gilmore, G

    Masseron, T. & Gilmore, G. Carbon, nitrogen andα-element abundances determine the forma- tion sequence of the Galactic thick and thin discs.Mon. Not. R. Astron. Soc.453, 1855–1866 (2015)

    2015

  12. [20]

    J.942, 35 (2023)

    Carrillo, A.et al.The Relationship between Age, Metallicity, and Abundances for Disk Stars in a Simulated Milky Way.Astrophys. J.942, 35 (2023)

    2023

  13. [21]

    & Nakasato, N

    Kobayashi, C. & Nakasato, N. Chemodynamical Simulations of the Milky Way Galaxy.As- trophys. J.729, 16 (2011)

    2011

  14. [22]

    Implications for stellar and Galactic chemical evolution.Astron

    Smiljanic, R.et al.The Gaia-ESO Survey: Sodium and aluminium abundances in giants and dwarfs. Implications for stellar and Galactic chemical evolution.Astron. Astrophys.589, A115 (2016)

    2016

  15. [23]

    E., Amarsi, A

    Nissen, P. E., Amarsi, A. M., Sk ´ulad´ottir, ´A. & Schuster, W. J. Abundances of iron-peak elements in accreted and in situ born Galactic halo stars.Astron. Astrophys.682, A116 (2024)

    2024

  16. [24]

    & Sk ´ulad´ottir, ´A

    Ernandes, H., Feuillet, D., Feltzing, S. & Sk ´ulad´ottir, ´A. Gaia-Sausage-Enceladus star forma- tion history as revealed by detailed elemental abundances.arXiv e-printsarXiv:2505.06606 (2025)

    2025

  17. [25]

    C., Vasiliev, E., Iorio, G., Evans, N

    Myeong, G. C., Vasiliev, E., Iorio, G., Evans, N. W. & Belokurov, V . Evidence for two early accretion events that built the Milky Way stellar halo.Mon. Not. R. Astron. Soc.488, 1235– 1247 (2019)

    2019

  18. [26]

    T.et al.The origin of accreted stellar halo populations in the Milky Way using APOGEE, Gaia, and the EAGLE simulations.Mon

    Mackereth, J. T.et al.The origin of accreted stellar halo populations in the Milky Way using APOGEE, Gaia, and the EAGLE simulations.Mon. Not. R. Astron. Soc.482, 3426–3442 (2019)

    2019

  19. [27]

    Belokurov, V .et al.The biggest splash.Mon. Not. R. Astron. Soc.494, 3880–3898 (2020). AcknowledgementsG.Y .W. gratefully acknowledges financial support from from the China Scholarship Council (CSC). This work has made use of the data from the DESI Member Institutions (https:/...

    2020

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