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REVIEW 3 major objections 4 minor 73 references

Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17

T0 review · 3 major / 4 minor · reviewed 2026-08-04 · deepseek-v4-flash

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Pith's one-line read M giant stars recover the Milky Way's known substructures and reveal metal-rich stars formed from gas delivered by the ancient Gaia-Enceladus merger.

desk verdict Solid catalog paper with honest limitations; overstates its conclusions in the abstract. read the letter →

arxiv 2509.07744 v1 pith:W5OW4OCB submitted 2025-09-09 astro-ph.GA

Longfei Ding , Jing Li , Xiang-Xiang Xue , Hao Tian , Zhengzhou Yan , Gang Zhao This is my paper · ORCID
classification astro-ph.GA
keywords MgiantsGalacticsubstructuresintegralsofmotionFriends-of-FriendsclusteringGaia-Enceladus-SausageSplashMilkyWayformationstellarpopulations
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 tries to establish that M giant stars—luminous, low-temperature red giants that resist extinction—are practical tracers for finding the Milky Way's ancient building blocks. Applying friends-of-friends clustering to 3,343 M giants selected from an updated spectroscopic catalog, with astrometry and chemical abundances, it recovers five known substructures: the Sagittarius stream, the Galactic Anticenter Substructure, the Gaia-Enceladus-Sausage (GES), Splash, and the high-alpha disk, plus two groups that do not match anything previously catalogued. Its central claim is chemical: some M giants inside the GES are metal-rich and split into two alpha-abundance sequences, which the authors read as stars formed from metal-enriched gas delivered by that ancient merger. If true, this adds an independent datapoint for the scenario in which the early Milky Way's high-alpha disk was later heated by the merger into the Splash population while the undisturbed remainder became today's high-alpha disk.

What carries the argument

The central machinery is the integrals-of-motion (IoM) space built from five orbital parameters—eccentricity, semi-major axis, orbital pole direction, and apocenter direction—computed from each star's full six-dimensional phase-space coordinates. In this space, the paper applies a friends-of-friends algorithm with an orbit-likelihood distance, so stars sharing similar orbits link into groups even when dynamical mixing has scrambled their current positions. The load-bearing tracer is the M giant itself: luminous and less affected by extinction, it reaches low Galactic latitudes and large distances, which is why the recovered groups extend earlier samples built from K giants, RR Lyrae stars, o

What would settle it

Recompute the clustering on the same 3,343 M giants using distances from an independent calibration, for example asteroseismic or spectrophotometric distances, and repeat the friends-of-friends linkage. Then check whether the 115 Gaia-Enceladus-Sausage members—especially the metal-rich ones with two alpha sequences—still occupy the high-eccentricity, low-angular-momentum locus. If the metal-rich group dissolves, shifts toward the thick-disk sequence, or changes its energy-angular momentum shape, the claim that these stars formed from merger-delivered gas is unsupported. A cheaper check: apply

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Extended reading notes

Core claim

The paper's central claim is that an M-giant sample, clustered in integrals-of-motion space, can both recover known Milky Way substructures and reveal the chemical signature of star formation fed by an ancient merger. Using five orbital parameters—eccentricity, semi-major axis, orbital pole direction, and apocenter direction—the authors compute an orbit-likelihood distance and apply friends-of-friends clustering to 3,343 M giants. They identify 47 groups totaling 1,597 stars: 182 in the Sagittarius stream, 594 in the Galactic Anticenter Substructure, 115 in the Gaia-Enceladus-Sausage, 121 in Splash, and 557 in the high-alpha disk. The GES members show the characteristic low-angular-momentum,

Load-bearing premise

The load-bearing premise is that the catalog's photometric distances—with roughly 25% relative uncertainty—are accurate enough that the computed orbital parameters separate true stellar groups rather than distance-error artifacts; the authors themselves replace those distances for Sagittarius stream members because they are noticeably underestimated.

Editorial extensions

If this is right

  • M giants can serve as standard tracers for low-latitude and distant Galactic substructures, complementing K giants, RR Lyrae stars, and BHB stars.
  • The GES sample is extended toward metal-rich M giants with two alpha sequences, supporting the idea that the GES progenitor delivered gas that formed new stars.
  • The GASS sample grows to 594 M giants, giving more leverage on the outer disk's structure and possible ripple patterns.
  • Splash and high-alpha disk samples provide an independent view of the early disk heating event: the same thick-disk chemistry appears in two different orbital states.
  • Two previously unknown groups with no clear counterpart suggest that M giants may reveal further structures not seen with other tracers.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the distance systematics that forced a correction for Sagittarius stream members also affect GES members, the metal-rich GES claim could be tested by re-running the clustering with an independent distance scale; the paper does not propagate these uncertainties.
  • The two unclassified groups might be fragments of known structures—Group 2 could be a metal-rich tail of the low-energy retrograde substructure Thamnos—or genuinely new groups; the paper only offers these as hypotheses.
  • Jointly using chemistry inside the clustering likelihood could suppress the low-alpha contamination the paper notes within the high-alpha disk group.
  • The M giants with |Z|<2 kpc, deliberately excluded here, could extend the same analysis into the disk plane where extinction matters most and where M giants are especially valuable.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 4 minor

Summary. The paper uses the LAMOST DR9 M-giant catalog of Li et al. (2023), distances from Qiu et al. (2023), Gaia DR3 proper motions, and APOGEE DR17 abundances to identify Milky Way substructures through a Friends-of-Friends clustering algorithm applied in integrals-of-motion space. The authors report members of the Sagittarius stream, GASS, GES, Splash, and the high-α disk, plus two groups without clear literature counterparts. They further claim that metal-rich M giants in the GES region show bimodal α-abundances, supporting the Zhao & Chen (2021) scenario in which the GES progenitor delivered metal-enriched gas, and that the identified Splash and high-α disk populations confirm a specific early-Milky-Way evolutionary picture.

Significance. If the structure identifications are robust, the paper provides a useful demonstration that M giants are viable tracers of known Galactic substructures in IoM space, and the metal-rich GES M giants with low- and high-α sequences add an independent observational datapoint to the debate about the GES progenitor's gas content and star formation. The external benchmarks against Belokurov et al. (2014), Hernitschek et al. (2017), Yang et al. (2019b), Li et al. (2021), and Tang et al. (2024) are a strength, and the machine-readable tables of member parameters are a useful community resource. However, the central quantitative claim—the association of individual M giants with specific substructures—rests on orbital parameters derived from photometric distances with ~25% relative uncertainty, and the paper does not demonstrate that the clustering is stable under realistic distance errors. The strongest wording of the conclusions, especially 'confirm the existence of metal-rich constituents within the GES' and 'confirm the evolutionary scenario,' goes beyond what the data and analysis can support.

major comments (3)
  1. [§2, §3, §4.1] The paper never quantifies how the ~25% relative distance uncertainty of Qiu et al. (2023) propagates into the orbital parameters used in Eq. (1) and into the FoF group assignments. Since the linking lengths are as small as 0.138 in normalized units, a star displaced by a 25% distance error (0.6–1.3 kpc at the relevant distances) can easily cross the boundaries between groups. The problem is not hypothetical: §4.1 states that the Qiu et al. distances for Sgr members are 'noticeably underestimated' and the authors replace them with a different calibration. That correction was applied after clustering for Sgr only, and the same systematic bias could affect the GASS, GES, Splash, or high-α memberships. A robustness test—e.g., re-running the FoF with distance realizations drawn from the quoted uncertainties, or a bootstrap over distance/velocity errors—is needed to show that the 115 GES, 121
  2. [§4.3, §5] The claim that metal-rich M giants 'confirm' the existence of metal-rich constituents within GES and the evolutionary scenario is not supported by the presented evidence. The GES sample has only 11 stars with reliable APOGEE [M/H] and [α/M] measurements, and the bimodal low-α/high-α separation is based on a visual inspection of Figure 6. No statistical test of bimodality, no significance estimate, and no contamination assessment from the background disk/halo are provided. With only 11 stars, the bimodal appearance could arise from small-number statistics or from overlap with Splash and thick-disk populations. The abstract and §5 should be tempered to 'consistent with' or 'suggest' rather than 'confirm'.
  3. [§4.1] The Sgr distance correction introduces an inconsistency in the analysis pipeline. Clustering in §3 used the original Qiu et al. (2023) distances, while §4.1 replaces the distances for Sgr members with the Li et al. (2016a) color-magnitude relation and then compares the corrected Sgr sample with literature data. It is not stated whether the orbital parameters and group membership were recomputed after the correction, nor how this correction affects the other substructures. This inconsistency weakens the reliability of the Sgr leading-arm decomposition and raises questions about the uniform application of the distance model across the full sample.
minor comments (4)
  1. [Abstract] The wording 'confirm the existence' and 'confirms the evolutionary scenario' is too strong for the data presented; consider using 'suggests' or 'supports'.
  2. [§3, Table 2] The linking length is selected individually for each group (Appendix A). The paper should state how many groups used each of the two extreme values and whether the results are stable to small changes in the linking length.
  3. [§4.5, Figure 9] The authors acknowledge low-α disk contamination in the high-α sample. It would be useful to quantify the contamination fraction, e.g., by comparing the [α/M] distribution of members with the field sample.
  4. [§4.6] Group 1 and Group 2 have only 15 and 13 members, respectively, with essentially no chemical data. The claim that Group 2 is a metal-rich extension of Thamnos is highly speculative and should be labeled as such.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the orbit-based grouping is benchmarked against external literature, and the GES chemical claim is not defined by the scenario it supports.

full rationale

The derivation chain is self-contained rather than circular. The M-giant sample and distances are taken from published catalogs (Li et al. 2023; Qiu et al. 2023), the orbital parameters entering the friends-of-friends clustering are computed from the explicit definitions in Section 3 and Eq. (1), and the resulting groups are labeled by comparing with externally defined substructures (Belokurov et al. 2014, 2018; Hernitschek et al. 2017; Naidu et al. 2020; Li et al. 2021; Tang et al. 2024). The metal-rich GES claim is not circular: the 11 APOGEE-matched GES M giants are selected purely from IoM clustering, not from the Zhao & Chen (2021) MRSK metallicity/kinematic cuts, so finding bimodal alpha abundances in that sample is an independent observation rather than a restatement of the input definition. The Sgr distance correction in Section 4.1 is a post-identification data correction, not a fitted parameter renamed as a prediction. Heavy self-citation exists (e.g., Li et al. 2023; Qiu et al. 2023; Wang et al. 2022; Yang et al. 2019b; Zhao & Chen 2021; and the in-preparation Xue et al. 2024 cited for the IoM parameter set), but these citations supply data, method, or interpretation; they do not by construction force the reported substructure memberships or the chemical conclusions. No equation in the paper reduces to another by construction, and no fitted quantity is presented as an independent prediction. The main vulnerabilities, such as ~25% distance uncertainties and small APOGEE-matched samples for GES, are correctness/robustness concerns rather than circularity.

Assumptions & free parameters 4 free parameters · 6 assumptions · 3 invented entities

The clustering rests on a chain of adopted inputs: literature constants for the solar position and motion, the Li et al. (2023) M giant catalog, the Qiu et al. (2023) distances with ~25% uncertainty, Gaia DR3 proper motions, and potential-model-dependent orbital parameters from an unspecified potential. The FoF hyperparameters (linking length, weights, minimum group size) are imported or tuned rather than derived, and the chemical interpretation relies on APOGEE subsets of 11 to 122 stars per component. No new physical forces or particles are required by the method itself; the two unclassified groups and the proposed two-component Sgr leading arm are candidate structures without independent confirmation.

free parameters (4)
  • FoF linking length per group = 0.138 to 0.487 (per-group value not listed)
    Clustering threshold that sets which stars join a group; chosen per group by inspecting member-count jumps (Appendix A); the exact value for each of the 47 groups is not reported.
  • Orbit-likelihood weights in Eq. 1 = adopted from Wang et al. (2022), values not given
    The four normalization weights set the relative scale of angular, semi-major-axis, eccentricity and apocenter differences; they control the clustering and are imported from a cited same-group paper without re-derivation.
  • Minimum FoF group size = 10 members
    Groups with fewer than 10 members are discarded; this affects recovery of sparse structures like the two unclassified groups and the Sgr debris component.
  • Sgr distance correction scale (Li et al. 2016a CMR) = not quantified
    Distances of all Sgr members are re-derived with a different color-magnitude relation because Qiu et al. (2023) distances are judged underestimated; the correction is never tabulated, so Sgr orbit parameters depend on it.
assumptions (6)
  • domain assumption Integrals of motion persist as valid labels for accretion remnants despite phase mixing
    Section 3 opening paragraph; the whole FoF-in-IoM strategy assumes energy and angular momentum separate real groups from field stars in the adopted potential.
  • domain assumption Five IoM parameters (e_c, a, l_orbit, b_orbit, l_apo) computed in a spherical potential without dynamical friction
    Section 3 first paragraph; definition source is cited as Xue et al. (2024, in preparation), an unpublished same-group paper, and no derivation of the spherical-potential assumption is given in this text.
  • domain assumption The unstated Milky Way potential used for E, Lz, a, e is accurate
    Section 3; no potential model is named, yet every orbital parameter and therefore every group depends on the assumed gravitational field.
  • domain assumption Photometric distances with ~25% uncertainty are accurate enough for clustering
    Sections 2 and 3; uncertainties are quoted but never propagated into orbit space; Section 4.1 shows the distances were wrong enough for Sgr members that a separate correction was required.
  • domain assumption Visual overlap with published members is sufficient to assign groups to known substructures
    Section 4; associations are made by comparing distributions in (l,b), Lz-E, Vr-Vphi and chemistry to literature samples, not by a quantitative membership or contamination test.
  • domain assumption APOGEE DR17 abundances for the matched subset are representative of each population
    Sections 4.1 to 4.5; chemistry rests on 19, 122, 11, 23 and 83 stars respectively, with no correction for which M giants happen to have APOGEE spectra.
invented entities (3)
  • Group 1 (15 M giants)
    purpose: Candidate new substructure, possibly disk-born
    Section 4.6; cannot be matched to known substructures; zero stars with APOGEE chemistry; only kinematics (near Splash in Lz-E but lower eccentricity, larger radial distance) suggest a disk origin.
  • Group 2 (13 M giants)
    purpose: Candidate new substructure or metal-rich tail of Thamnos
    Section 4.6; slight retrograde motion and low energy match Thamnos kinematically, but the 2 stars with APOGEE abundances are more metal-rich than classical Thamnos; authors leave the association open.
  • Two-component Sgr leading stream
    purpose: Proposed division of the leading arm into a vertically crossing and a disk-parallel component
    Section 4.1 and Figure 4; based on 67 leading-arm members separated by color in Sgr longitude; the paper defers confirmation to future analysis and the two components are identified in the same data that produced the claim.

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Cite this review

Pith. "Pith review of Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17." pith.science (2026). https://pith.science/paper/W5OW4OCB

@misc{pith2026250907744,
  author       = {Pith},
  title        = {Pith review of: Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/W5OW4OCB}},
  note         = {Machine review of arXiv:2509.07744}
}
read the original abstract

Based on the updated M giant star catalog selected from LAMOST DR9, we iden-tify substructures within the integrals-of-motion space through Friends-of-Friends cluster-ing algorithm. We obtain members belonging to several known substructures: the Sagittarius stream, Galactic Anticenter Substructure (GASS), Gaia-Enceladus-Sausage (GES), Splash, and the high-{\alpha} disk. Furthermore, we also identify two groups which cannot be clearly asso-ciated with previously known substructures. Our findings confirm the existence of metal-rich constituents within the GES, representing newly formed stars that originated from the metal-enriched gas delivered during the GES merger event and subsequently evolved. Additionally, this study further expands the sample of GASS, high-{\alpha} disk, and Splash stars. Analysis of these metal-rich M giant stars as members of the GES, Splash, and high-{\alpha} disk compo-nents supports an evolution scenario for the early Milky Way, as proposed by previous stud-ies. In this scenario, stars initially formed in a high-{\alpha} primordial disk were dynamically heated by the massive accretion event (GES). This process redistributed stellar orbits, creat-ing the Splash population, while the undisturbed portion of the primordial disk persisted as the present-day high-{\alpha} disk component.

Figures

Figures reproduced from arXiv: 2509.07744 by the authors.

Figure 1
Figure 1. The distribution of the selected M giant sample in phase space: Galactic coordinates (l, b) (upper panel) and Xgc − Zgc plane (lower panel). Considering the substantial number of disk stars in our sample, to minimize the impact of the thin disk and thick disk, we only include samples with |Zgc| > 2 kpc in the clustering analysis. In future work, we will further analyze the M giant sample with |Zgc| < 2 kpc. Addition… view at source ↗
Figure 2
Figure 2. Since M giants lack reliable chemical abundance parameters, we used K giants from Yang et al. [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 2
Figure 2. The distribution of the selected M giant sample’s velocities (Vr, Vϕ, Vθ) and [M/H] relative to the Galactocentric distance (rgc). (2019b) as the disk star sample. These stars were cross-matched with APOGEE DR17 to derive [M/H] and [α/M] for comparison with the substructures [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figures from the paper (8 more)
Figure 3
Figure 3. Figure 3: The distributions of groups associated with known substructures in the Galactic coordinate system and the Lz￾E plane. The figure includes the known substructures Sgr (red pentagram), GASS (orange plus), GES (purple triangle), Splash (blue diamond), and High-α Disk (yel…
Figure 4
Figure 4. Figure 4: The distributions of the Sgr members in the Λ˜⊙-Vlos, Λ˜⊙-d, Xgc-Zgc, [M/H]-[α/M], Lz-E, and Ly-E planes. Λ˜⊙ is the longitude in the Sgr coordinate system and the definition is the same as that in Belokurov et al. (2014). The red, blue, and green points in the figure …
Figure 5
Figure 5. Figure 5: Distribution of our GASS members in (l, b), (Xgc, Ygc), (Ygc, Zgc), (Lz, E), (rgc, ec), and ([M/H], [α/M]) spaces. The blue and red points represent the members in the northern and southern hemispheres respectively. The silver points represent all M giant stars. In the…
Figure 6
Figure 6. Figure 6: Distribution of the GES in the Lz-E, Vr-Vϕ, rgc-ec, and [M/H]-[α/M] planes, as well as histograms of R and |Z|. The blue points represent all the selected GES member stars, while the silver points represent all M giant stars. In the middle-right panel, the blue and red…
Figure 7
Figure 7. Figure 7: The distribution of the angular momentum (Lz) and eccentricity (ec) of GES members. The blue solid line traces the distribution of the GES members in our sample, while the red dashed line traces the distribution of HAC K giants from Yang et al. (2019a) [PITH_FULL_IMAG…
Figure 8
Figure 8. Figure 8: Distribution of the Splash in the Lz-E, Vr-Vϕ, rgc-ec, and [M/H]-[α/M] planes. The blue points and silver points represent the Splash members and all M giants, respectively. In the bottom-right panel, the blue stars represent 23 Splash M giants, while the yellow points…
Figure 9
Figure 9. Figure 9: Distribution of high-α disk stars in the Lz-E, Vr-Vϕ, rgc-ec, and [M/H]-[α/M] planes. The blue points represent high-α disk members, while the silver points represent all M giants. In the bottom-right panel, the blue stars represent 83 high-α disk stars, while the yell…
Figure 10
Figure 10. Figure 10: The distribution of the unclassified groups in the chemodynamical space. The red and blue points in the figure represent Group 1 and Group 2, respectively. The silver points represent all M giants. In the bottom-right panel, the blue stars represent stars from Group 2…

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