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arxiv: 2606.06594 · v2 · pith:SASD7J42new · submitted 2026-06-04 · 🌌 astro-ph.GA

The accretion history of the Milky Way V. The kinematics of most globular clusters trace the merger epochs

Istiak Akib , Fran\c{c}ois Hammer , Yanbin Yang This is my paper

Pith reviewed 2026-06-28 00:14 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords globular clustersGaia-Sausage-EnceladusMilky Way accretionenergy-angular momentum planeN-body simulationsmerger kinematicsorbital energy loss
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The pith

Simulations of the Milky Way-GSE merger show that most globular clusters linked to the event retain enough orbital energy to be identified with accretion in the E-Lz plane.

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

The paper examines whether globular clusters can still serve as reliable markers of the Gaia-Sausage-Enceladus merger after accounting for orbital energy changes. Using three N-body simulations that include GCs from the GSE disc, its halo, and those formed during the merger itself, the authors find that disc-origin clusters lose substantial energy through repeated passages in the dense galactic disc while halo and in-merger clusters keep energies comparable to GSE stars even nine billion years later. This leads to the conclusion that the majority of observed GCs associated with GSE can still be matched to the accretion event when both energy-angular momentum position and age-metallicity information are combined. A reader would care because reconstructing the Milky Way's merger timeline depends on correctly linking present-day clusters to past events.

Core claim

By modelling three N-body simulations of the Milky Way-GSE merger that vary initial masses, mass ratios and gas content, and by including GC populations from the GSE disc progenitor, its halo and those formed in the merger, the work shows that disc GCs lose orbital energy while halo and merger-formed GCs largely retain it. Consequently most GCs observationally linked to GSE remain associable with the accretion event in the E-Lz plane, supporting earlier identifications that combine kinematics with age-metallicity relations.

What carries the argument

The energy-angular momentum (E-Lz) plane, with the central mechanism being the differential orbital energy loss experienced by GSE disc GCs (via disc passages and possible tidal shocks) versus the retention of energy by halo GCs and those formed during the merger.

Load-bearing premise

The three N-body simulations with varied initial masses, mass ratios and gas content are sufficient to represent the real dynamical evolution and survival rates of globular clusters during the Milky Way-GSE merger.

What would settle it

A direct count showing that the majority of observed GSE-associated globular clusters have orbital energies well below the range retained by the simulated halo and merger-formed populations would falsify the claim.

Figures

Figures reproduced from arXiv: 2606.06594 by Fran\c{c}ois Hammer, Istiak Akib, Yanbin Yang.

Figure 1
Figure 1. Figure 1: Time evolution of the merger process for Model 1. Left, middle columns: Projections of the simulated GC positions onto the [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Time evolution of the merger process for Model 2. Left, middle columns: Projections of the simulated GC positions onto [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Distributions of stars and GCs of the three models in the [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
read the original abstract

Several studies have associated globular clusters (GCs) with former Galactic accretion events by comparing their positions in the energy-angular momentum ($E$-$L_z$) plane, an approach further supported by similarities in their age-metallicity relations. However, recent merger simulations suggest that GCs initially associated with the Gaia-Sausage-Enceladus (GSE) disc may have lost their orbital energy and thus may not reliably trace this accretion event. We extend this framework by considering three N-body simulations of the Milky Way-GSE merger with different initial masses, mass ratios, and gas content. In addition to GCs belonging to the GSE disc progenitor, we accounted for GCs in its halo and, in gas-rich models, a population of GCs formed during the Milky Way-GSE merger. We confirm that most GCs originating in the disc have lost a significant part of their orbital energy during repeated passages through the dense disc medium, and we conjecture that associated tidal shocks may have destroyed many of them. In contrast, GCs from the halo and GCs formed during the merger have largely retained their orbital energy, which remains comparable to that of GSE stars even up to 9 Gyr after the completion of the merger. By using a more realistic GC population and GSE modelling, we find that most GCs linked to GSE can be associated with Milky Way accretion events in the $E$-$L_z$ plane, which supports previous observational associations based on a combination of energy-angular momentum and age-metallicity relations.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 0 minor

Summary. The paper uses three N-body simulations of the Milky Way-GSE merger that vary initial masses, mass ratios, and gas content. In addition to disc GCs, it includes halo GCs from the GSE progenitor and, in gas-rich cases, merger-formed GCs. It finds that disc GCs lose orbital energy through repeated disc passages while halo and merger-formed GCs largely retain energy comparable to GSE stars even 9 Gyr post-merger. The central claim is that this more realistic GC population allows most observed GSE-linked clusters to be associated with accretion events in the E-Lz plane, supporting prior observational links based on E-Lz and age-metallicity relations.

Significance. If the result holds, the work strengthens the reliability of the E-Lz plane for associating Milky Way globular clusters with specific accretion events and helps reconcile simulation-based concerns about energy loss with observational associations. The explicit inclusion of halo and in-situ merger-formed populations across multiple realizations is a positive step beyond single-setup models.

major comments (2)
  1. [Abstract and simulation setup] Abstract and simulation-setup paragraph: The claim that halo and merger-formed GCs dominate the surviving GSE-linked population (and thus that most observed associations remain valid) rests on only three N-body runs that vary initial masses, mass ratios, and gas content. No fractions of surviving GCs per component are reported, and no sensitivity tests are described for other parameters such as orbital inclination or gas fraction at pericentre; without these, it is unclear whether the energy-retention result is generic or specific to the chosen setups.
  2. [Results] Results discussion: The statement that halo and merger-formed GCs 'have largely retained their orbital energy' and 'remain comparable to that of GSE stars' is presented without quantitative comparison (e.g., histograms, median E values, or Kolmogorov-Smirnov tests) between the simulated E-Lz distributions and the observed distribution of GSE-associated clusters; such a direct comparison is needed to substantiate that 'most GCs linked to GSE can be associated' in the E-Lz plane.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments and positive assessment of the significance of our work. We address each major comment below.

read point-by-point responses

  1. Referee: [Abstract and simulation setup] Abstract and simulation-setup paragraph: The claim that halo and merger-formed GCs dominate the surviving GSE-linked population (and thus that most observed associations remain valid) rests on only three N-body runs that vary initial masses, mass ratios, and gas content. No fractions of surviving GCs per component are reported, and no sensitivity tests are described for other parameters such as orbital inclination or gas fraction at pericentre; without these, it is unclear whether the energy-retention result is generic or specific to the chosen setups.

    Authors: We agree that explicitly reporting the fractions of surviving GCs per component (disc, halo, and merger-formed) will improve clarity and will add these values to the revised manuscript. The three simulations already vary initial masses, mass ratios, and gas content, with consistent energy retention for halo and merger-formed GCs across all runs. We acknowledge that further sensitivity tests (e.g., orbital inclination or pericentric gas fraction) would be desirable but would require additional simulations beyond the scope of this study. revision: partial

  2. Referee: [Results] Results discussion: The statement that halo and merger-formed GCs 'have largely retained their orbital energy' and 'remain comparable to that of GSE stars' is presented without quantitative comparison (e.g., histograms, median E values, or Kolmogorov-Smirnov tests) between the simulated E-Lz distributions and the observed distribution of GSE-associated clusters; such a direct comparison is needed to substantiate that 'most GCs linked to GSE can be associated' in the E-Lz plane.

    Authors: We will incorporate quantitative comparisons in the revised manuscript, including histograms of orbital energy distributions, median E values for each GC component, and direct overlays against the observed GSE-associated clusters to strengthen the substantiation of our claims. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained via new simulations

full rationale

The paper's central claim rests on three new N-body simulations of the MW-GSE merger (varying initial mass, mass ratio, and gas content) whose outputs are compared directly to observed GC positions in the E-Lz plane. No load-bearing step reduces a prediction to a fitted parameter from the same dataset, redefines a quantity in terms of itself, or relies on a self-citation chain whose validity is presupposed by the present work. The simulations are treated as independent dynamical experiments whose results are then juxtaposed with external observations; the derivation chain therefore remains externally falsifiable and does not collapse by construction.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of N-body dynamics and the representativeness of three chosen merger models; no new entities are postulated.

free parameters (2)
  • initial masses and mass ratios
    Varied across the three simulations to explore different merger scenarios.
  • gas content
    Included or varied in gas-rich models to affect GC formation and dynamics.
axioms (1)
  • domain assumption N-body simulations accurately capture the orbital energy evolution of GCs during repeated disc passages and tidal shocks.
    Invoked to explain energy loss in disc GCs versus retention in halo and merger-formed populations.

pith-pipeline@v0.9.1-grok · 5819 in / 1266 out tokens · 27407 ms · 2026-06-28T00:14:55.861188+00:00 · methodology

discussion (0)

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Reference graph

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