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REVIEW 3 major objections 6 minor 1 cited by

Evidence of Gaia Enceladus experiencing at least two passages around the Milky Way

T0 review · 3 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read The Milky Way's last giant merger crossed the Galaxy at least twice before dissolving.

desk verdict Plausible new interpretation of the Nissen-Schuster accreted halo split as multiple Gaia Enceladus passages, with new Al abundances, but the load-bearing energy-to-epoch mapping comes from one idealized simulation and the sample is small. read the letter →

arxiv 2506.00409 v1 pith:K7RKU7IU submitted 2025-05-31 astro-ph.GA astro-ph.SR

Ása Skúladóttir , Heitor Ernandes , Diane K. Feuillet , Alice Mori , Sofia Feltzing , Romain E. R. Lucchesi , Paola Di Matteo This is my paper
classification astro-ph.GAastro-ph.SR
keywords GaiaEnceladusgalaxymergersstellarchemicalabundancesaluminumorbitalenergygalacticarchaeologyN-bodysimulations
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

This paper claims that the last major galaxy to merge with the Milky Way, Gaia Enceladus, did not fall in on a single trajectory but made at least two close passages before being shredded into the halo. The authors split a long-studied sample of accreted halo stars into two groups by present-day orbital energy: high-energy stars carry chemical signatures of a less efficient star-forming environment, as expected for stars stripped from Gaia Enceladus's outskirts on early passages, while low-energy stars trace the more enriched inner regions that fell in later. The chemical separation appears consistently in $[\mathrm{Al/Fe}]$, $[\mathrm{Mg/Fe}]$, and $[\mathrm{Mg/Ba}]$, with new aluminum measurements playing a decisive role. If the interpretation holds, earlier single-passage estimates of the galaxy's mass, metallicity, and merger timescale will need revision.

What carries the argument

The load-bearing tool is a mapping from present-day orbital energy and angular momentum (the $E$\textendash$L_z$ plane) to the star's original location and stripping time in the progenitor, taken from the authors' dissipationless N-body simulations of a 1:10 mass-ratio merger. Using this mapping, they set the energy threshold $E_{\rm cut}$ at $-4.5 \times 10^4\, \mathrm{km^2\,s^{-2}}$ to separate stars stripped during early (outer galaxy) versus late (inner galaxy) passages. The chemical abundances $[\mathrm{Al/Fe}]$, $[\mathrm{Mg/Fe}]$, and $[\mathrm{Mg/Ba}]$ then serve as tracers of star formation efficiency, with the new high-precision aluminum measurements from high-resolution VLT/UVES spectra helping to separate accreted from in-situ stars.

What would settle it

Recompute the orbital energies for the 33 accreted stars using an independent Milky Way potential model or an independent simulation suite, re-apply the energy cut, and re-run the Kolmogorov-Smirnov tests on [Mg/Fe] and [Mg/Ba] at [Fe/H] > −1; if the separations no longer hold at p < 0.01, the claimed two-passage signature would be a modeling artifact rather than a property of the stars.

Watch

Extended reading notes

Core claim

The paper's central claim is that the two chemically distinct groups of low-$\alpha$ stars in the solar neighborhood do not require two separate mergers (Thamnos or Eos) but are two accretion epochs of the same galaxy. Guided by N-body simulations, the authors identify stars with orbital energy above $E_{\rm cut} = -4.5 \times 10^4\, \mathrm{km^2\,s^{-2}}$ as material stripped from the outer regions of Gaia Enceladus during early passages, and lower-energy stars as material from the inner regions accreted later. Across $[\mathrm{Al/Fe}]$, $[\mathrm{Mg/Fe}]$, and $[\mathrm{Mg/Ba}]$, the high-energy group shows the lower enrichment efficiency expected from the outskirts, and the low-energy group has on average higher $[\mathrm{Fe/H}]$. The paper concludes that Gaia Enceladus experienced at least two passages before dispersal, presenting what it describes as the first observational evidence for a multi-passage merger history of this event.

Load-bearing premise

The mapping from a star's present-day orbital energy to the passage in which it was stripped and its original position in Gaia Enceladus, taken from the authors' simulations and the assumed Milky Way potential, is correct; if it is not, the observed chemical differences between the high- and low-energy groups could have another cause.

Editorial extensions

If this is right

  • Gaia Enceladus made at least two passages through the Milky Way before full disruption.
  • High-energy accreted stars were stripped first, from the outer, less chemically evolved regions; low-energy stars came later from the inner regions.
  • The chemical and kinematic data can be explained by a single progenitor with an internal star-formation gradient, removing the need to invoke a separate Thamnos or Eos merger for this sample's low-energy stars.
  • Pictures of Gaia Enceladus built only from its high-energy (outskirt) stars underestimate the galaxy's metallicity, stellar mass, and the duration of the merger event.
  • Simulated energy-angular-momentum overdensities from individual passages should be detectable with larger samples, allowing stars to be assigned to specific passages.

Reading between the lines

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

  • The method could be applied to other proposed accreted structures: a similar energy split in chemically indistinguishable stars would hint at multi-passage infall rather than separate progenitors.
  • A larger, magnitude-limited sample of accreted stars with high-quality spectra would test whether the abundance trends vary continuously with energy, as expected if each passage strips a slightly different layer of the progenitor.
  • If the passages are real, reconstructions of the Gaia Enceladus merger should include a time-resolved infall, which would change estimates of when the last major merger ended.
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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 / 6 minor

Summary. The paper investigates whether the accreted stellar population associated with Gaia Enceladus (the Nissen & Schuster halo sample) contains stars stripped during at least two distinct orbital passages. The authors add new high-resolution VLT/UVES aluminum abundances for 49 stars, combine them with published [Mg/Fe], [Mg/Ba], and other abundance ratios, and split the 33 accreted stars with known orbital energies into high-energy and low-energy subsamples using a threshold E_cut = −4.5×10^4 km^2/s^2 motivated by their own N-body simulation. They report that high-energy stars have systematically lower [Mg/Fe] and a steeper [Mg/Ba] trend at [Fe/H] > −1 (KS tests p < 0.01), and a lower mean [Al/Fe] (though the mean difference is only 0.04 dex and within 2σ). The paper concludes that high-energy stars were stripped from the outskirts of Gaia Enceladus during early passages, while low-energy stars came from its inner regions at later passages, providing the first observational evidence of multiple passages.

Significance. If the interpretation is correct, the paper opens a new observational window onto the accretion history of Gaia Enceladus and demonstrates that 'the merger' should be viewed as a sequence of stripping events, with implications for recovered stellar mass, metallicity, and merger timescales. The new aluminum measurements are a valuable addition to the sample, and the paper is commendably explicit about its limitations: the small sample size, the approximate nature of the energy threshold, and the need for independent simulations. The robustness tests in Appendix A show that the chemical differences do not depend sensitively on the exact value of E_cut. However, the central claim rests entirely on a single dissipationless N-body simulation (Mori et al. 2024) with a fixed mass ratio and orbital setup, and the paper does not test alternative Milky Way potentials or independent merger simulations. Because the energy-to-accretion-epoch mapping is the load-bearing assumption, the present evidence is a well-characterized energy–chemistry correlation rather than a direct, model-independent proof of two passages.

major comments (3)
  1. [Sec. 3 and Fig. 1] The interpretation that high present-day orbital energy corresponds to early accretion from the outskirts of Gaia Enceladus is taken entirely from the authors' own N-body simulation (Mori et al. 2024), which adopts a fixed 1:10 mass ratio and one set of orbital parameters. The energy threshold E_cut = −4.5×10^4 km^2/s^2 is derived from this simulation, and all energies are computed in MWPotential2014. Appendix A varies E_cut but not the adopted Milky Way potential or the simulation mapping itself. A different choice of Galactic potential (e.g., one with a different halo mass or flattening) can shift the energy scale and reassign stars between the high- and low-energy bins, while an independent simulation with different initial conditions could change the predicted energy–radius–accretion epoch relation. The central claim therefore requires either a test with independent simulations or a demonstration that the chemical differences persist under alternative energy definitions. As written, the evidence is an energy–chemistry correlation whose discrete-passage interpretation is model-dependent.
  2. [Sec. 4 and Fig. 3] The [Al/Fe] evidence is weaker than the abstract and Section 5 imply. The mean difference between the high- and low-energy samples is only 0.04 dex (⟨[Al/Fe]⟩_HE = −0.67±0.01 vs. ⟨[Al/Fe]⟩_LE = −0.63±0.03), which lies within 2σ of the error of the mean. The KS test gives p < 0.05, but no effect size or confidence interval for the mean difference is reported, and the subsamples contain only 9 and 13 stars. This result alone would not support a statistically meaningful separation, and the statement that 'in all observed abundance ratios' the two populations differ is therefore overstated. The authors should either report a proper confidence interval for the [Al/Fe] difference or explicitly downgrade the [Al/Fe] claim to suggestive rather than robust.
  3. [Sec. 4, bottom panel of Fig. 3] The exclusion of the Ba-rich star with [Mg/Ba] < 0 from the low-energy sample is post hoc and is justified only by the general phenomenon of binary mass transfer from AGB companions, not by any evidence that this particular star is binary. Because the reported KS test for [Mg/Ba] at [Fe/H] > −1 (p < 0.01) depends on this exclusion, the authors should either demonstrate with independent data (e.g., radial velocity variability or a distinctive s-process abundance pattern) that this star is a binary, or show that the statistical significance and the slope difference persist when the star is included. Without such a test, the [Mg/Ba] claim is not robust to a single data point.
minor comments (6)
  1. [Title and Sec. 1] The title contains a clear typo: 'Milky W ay' should be 'Milky Way'.
  2. [Sec. 1] The timescales for ccSN and SNIa enrichment are quoted as '∼10^7 Gyr' and '∼10^9 Gyr'; given the context these should be years (10^7 yr and 10^9 yr), not gigayears.
  3. [Throughout] The notation for the energy threshold is inconsistent: the text uses 'E cut', 'E_cut', and 'Ecut' in different places. Please use a single notation consistently.
  4. [Appendix B] There is a typo in 'one-dimentional' which should be 'one-dimensional'.
  5. [Sec. 2.1] The description of the ESO archive search ('spectra of comparable quality') is vague; please state the criteria used to select the eight additional stars, or the number of spectra examined, so that the sample selection is reproducible.
  6. [Fig. 2 caption] The caption states that point size increases with decreasing [Fe/H], but the figure does not show a scale; consider adding a legend or a note specifying the size coding.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the chemical abundances are independent data that test the simulation-motivated energy split.

full rationale

The paper's central observable evidence is not circular. The accreted stars are divided at E_cut = -4.5e4 km^2/s^2, a threshold motivated by the authors' N-body simulations (Mori et al. 2024, Fig. 1), which also supply the mapping from present-day orbital energy to accretion epoch and original radius in the progenitor. However, the chemical abundances ([Al/Fe], [Mg/Fe], [Mg/Ba], etc.) come from new UVES spectra and from the published NS sample; they are not used to define the energy groups and are not fitted to the threshold. The observed abundance differences between high- and low-energy stars are therefore independent measurements that could have falsified the simulation-based expectation. The robustness test in Appendix A varies E_cut by +/-1e4 km^2/s^2 and finds the same qualitative abundance trends, showing that the conclusion is not an artifact of the exact cut. The main self-citation (Mori et al. 2024, with overlapping authors Mori and Di Matteo) is prior, externally checkable N-body work with stated assumptions, not an imported uniqueness theorem, and the qualitative expectation that loosely bound outer stars are stripped first is supported by other cited simulations and physical reasoning. The paper explicitly flags the modeling limitations: the quantitative energy scale may depend on simulation scaling and on the adopted MWPotential2014, and it calls for independent simulations to establish the robustness of the theoretical predictions. These are validity risks, not circular steps. No equation is defined in terms of the conclusion, and no fitted quantity is renamed as a prediction. Hence no significant circularity.

Assumptions & free parameters 2 free parameters · 6 assumptions · 0 invented entities

The central claim rests on (i) an energy threshold chosen from the authors' N-body simulations, (ii) an abundance cut for population classification, and (iii) a series of domain assumptions about the Milky Way potential, 1D LTE abundance analysis, sample representativeness, and the physical interpretation of chemical abundance ratios. No new physical entities are introduced. The main burden is that the simulation-based mapping from energy to accretion history is not independently validated.

free parameters (2)
  • E_cut (energy threshold) = -4.5e4 km^2/s^2
    Threshold used to split the accreted low-alpha stars into high- and low-energy samples. Chosen to match the authors' N-body simulation prediction (Mori et al. 2024, Sec. 3). The paper tests robustness to +/-1e4 km^2/s^2 (Appendix A), but the value is a hand-set boundary.
  • [Al/Fe] low-alpha cut = -0.5
    Abundance cut used to reclassify stars into accreted (low-alpha) versus in-situ (high-alpha) populations at low [Fe/H], where [Mg/Fe] separation is small. Used in Sec. 4 to include/exclude stars from the accreted sample; value is chosen from the observed separation in the new Al data.
assumptions (6)
  • domain assumption The mapping from present-day orbital energy to accretion time and original location in the progenitor (outer stars stripped first, with higher energy) is correctly given by the authors' N-body simulations.
    Invoked in Sec. 3 and Fig. 1: 'From Fig. 1 we conclude that the stars that were accreted first resided in the outer regions of Gaia Enceladus and will typically have higher orbital energy after joining the Milky Way.' This is taken from Mori et al. 2024, which shares two co-authors with this paper. If this mapping fails, the interpretation of the energy split as multiple passages falls apart.
  • domain assumption The adopted Milky Way potential (MWPotential2014) and the Staeckel approximation give sufficiently accurate energies for halo stars in the solar neighborhood.
    Sec. 2.1: energies are computed with galpy using MWPotential2014 and actionAngleStaeckel. The paper notes the quantitative energy scale depends on the potential model, but does not test alternatives.
  • domain assumption 1D LTE model atmospheres with negligible non-LTE corrections for Al (tested < 0.02 dex) and published non-LTE-corrected abundances for Mg and Ba are adequate.
    Sec. 2.1: TURBOSPEC with MARCS 1D LTE models; non-LTE tests for Al show <0.02 dex; [Mg/Fe] and [Mg/Ba] are adopted from Nissen et al. (2024, 2011) with adopted uncertainties.
  • domain assumption The NS sample is a representative sample of the accreted halo population in the solar neighborhood, with no selection bias between the high- and low-energy groups.
    The analysis uses the historical Nissen & Schuster sample; the paper does not quantify how the original target selection affects the energy distribution of the accreted stars.
  • ad hoc to paper The excluded Ba-rich star is chemically peculiar due to binary mass transfer and is not representative of the low-energy population.
    Sec. 4: 'we have excluded one Ba-rich star ([Mg/Ba]<0) with low kinetic energy' because Ba is typically enhanced by AGB binary transfer. This is a physical justification, but the exclusion is applied only in the [Mg/Ba] panel and only to the low-energy sample.
  • domain assumption The extremely similar abundance trends of the two subpopulations imply a common progenitor rather than two separate galaxies.
    Sec. 5, argument 3: 'It is highly improbable that two separate galaxies independently experienced chemical enrichment that was so extremely similar...' This is a prior probability judgment, not a quantitative derivation.

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Cite this review

Pith. "Pith review of Evidence of Gaia Enceladus experiencing at least two passages around the Milky Way." pith.science (2026). https://pith.science/paper/K7RKU7IU

@misc{pith2026250600409,
  author       = {Pith},
  title        = {Pith review of: Evidence of Gaia Enceladus experiencing at least two passages around the Milky Way},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/K7RKU7IU}},
  note         = {Machine review of arXiv:2506.00409}
}
read the original abstract

One of the major recent breakthroughs has been the discovery of the last Major Merger to happen in the history of the Milky Way. Around 10 Gyr ago the galaxy Gaia Enceladus, with estimated ~10% of the Milky Way mass, fell into its potential, bringing a large amount of stars which can be identified through their unique chemical and kinematic signatures. Simulations have long predicted that a galaxy of this size should experience several passages through the disk of the Milky Way before eventually being fully dispersed. For the first time, we present observational evidence to support this. We identify two subpopulations accreted from Gaia Enceladus: 1) stars which today have large kinematic energy, which originate from the outskirts of Gaia Enceladus and were accreted during early passages; 2) stars with low kinetic energy accreted at later passages, originating from the inner parts of Gaia Enceladus. Through the use of high-precision chemical abundances, crucially including new aluminum measurements, we show that in all observed abundance ratios ([Fe/H], [Al/Fe], [Mg/Fe] and [Mg/Ba]), stars with high energy show evidence of coming from a less chemically evolved outer region of Gaia Enceladus, compared to the stars with low energy. We therefore conclude that Gaia Enceladus experienced several passages before merging with the main body of our Galaxy. This discovery has wide implications for our understanding of this event, and consolidates Gaia Enceladus as a benchmark for studying galaxy mergers and hierarchical galaxy formation in extraordinary details.

Figures

Figures reproduced from arXiv: 2506.00409 by the authors.

Figure 1
Figure 1. Simulation of Gaia Enceladus falling into the Milky Way (A. Mori et al. 2024). Top panels show the energy and angular momentum (E − Lz) relation for the accreted stars (small points) which are now residing in the Milky Way. Left panel shows all stars, and the right panel those at a sun-like distance from the Galactic center (7-9 kpc). Color-coding at a given point in (E − Lz) shows the average position of stars in G… view at source ↗
Figure 2
Figure 2. shows the energy and angular momentum of the NS sample. The energy range, is in generally good agreement with simulations, see [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. (middle panel). Based on our new data, we de￾fine a cut between the high- and low-α populations at [Al/Fe] = −0.5, resulting in 9 stars with Al measure￾ments in the low-energy sample and 13 with high en￾ergy. Our results show that the separation between the accreted and in-situ populations in [Al/Fe] is very clear (≳ 0.3 dex), even at the lowest [Fe/H], see [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Schematic figure showing how Gaia Enceladus fell into the potential of the Milky Way. First to be accreted with high kinetic energies are the outermost stars of Gaia Enceladus (green), from regions with less efficient star formation. These stars can subsequently be rec…
Figure 5
Figure 5. Figure 5: Same as [PITH_FULL_IMAGE:figures/full_fig_p011_5.png]
Figure 6
Figure 6. Figure 6: Chemical abundances for the NS accreted stellar sample with low (pink circles) and high (green triangles) energies, as defined in [PITH_FULL_IMAGE:figures/full_fig_p012_6.png]

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Reviewed August 7, 2026 · model on record in the stance chip above.