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arxiv: 2511.17060 · v2 · submitted 2025-11-21 · 🌌 astro-ph.GA

Comparing galaxy merger orbits in hydrodynamical simulation and in dark-matter-only simulation

Yahan Pu , Lan Wang , Guangquan Zeng , Lizhi Xie This is my paper

Pith reviewed 2026-05-17 20:58 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords galaxy mergershydrodynamical simulationsdark-matter-only simulationsmerger orbitsbaryonic effectsmerger timescalesorbital evolution
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The pith

Baryons cause galaxy mergers to complete earlier and follow more spiral-in orbits than in dark-matter-only simulations.

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

This paper compares galaxy mergers in simulations that include gas and stars with those that only model dark matter to see how baryons change the process. Matched pairs of merging galaxies are tracked from infall to final merger in both types of runs. The hydrodynamical versions show similar infall times but earlier merger completions, leading to shorter timescales overall. Merger orbits in the final stages are more head-on when baryons are present, with collision angles 6 to 10 degrees smaller. A sympathetic reader would care because merger dynamics shape how galaxies grow, form stars, and evolve over time.

Core claim

Compared with the mergers in the dark-matter-only simulation without baryons, the matched mergers in the hydrodynamical simulation have similar infall time, but have statistically earlier merger times and therefore shorter merger timescales. The merger orbits for the matched pairs are similar right after infall, and both evolve to more head-on orbits at final stages, with smaller changes in the hydrodynamical simulation. In the final 2 Gyr before merger, the collision angles are smaller in the hydro run by around 6 to 10 degrees, depending on mass ratios and galaxy masses.

What carries the argument

Comparison of matched merger pairs tracked across hydrodynamical and dark-matter-only versions of the same simulation volume, measuring differences in infall times, merger times, and collision angles.

Load-bearing premise

The matched merger pairs identified across the two simulation types are comparable enough that differences arise mainly from baryonic physics rather than from matching methods or resolution effects.

What would settle it

Finding no systematic difference in merger times or final collision angles between hydrodynamical and dark-matter-only runs in an independent simulation suite with similar matching would challenge the central claim.

Figures

Figures reproduced from arXiv: 2511.17060 by Guangquan Zeng, Lan Wang, Lizhi Xie, Yahan Pu.

Figure 1
Figure 1. Figure 1: Comparison of infall time (tinfall, left panel), merger time (tmerger, middle panel), and merger timescale (tinfall − tmerger, right panel) between matched merger pairs of TNG100-1 and TNG100-1-Dark. In each panel, the grey scale indicates the number of merger events included in each pixel in the figure with timestep of 0.24 Gyr (infall time), 0.16 Gyr (merger time), and 0.18 Gyr (timescale), respectively.… view at source ↗
Figure 2
Figure 2. Figure 2: Two examples of matched major mergers for massive gal [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Distributions of the collision angle of mergers for t [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: For the 178 matched major merger pairs with [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The distributions of collision angle for the matched [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: The same as Fig. 5, but for matched merger samples sele [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: The same as Fig. 5, but for matched merger samples sele [PITH_FULL_IMAGE:figures/full_fig_p013_7.png] view at source ↗
read the original abstract

To investigate how the presence of baryons in simulations affects galaxy merger orbits, we compare in detail the merger timescales and orbits of the matched merger pairs in TNG100 hydrodynamical simulations and their corresponding dark-matter-only simulations, for different resolution levels. Compared with the mergers in the TNG100-1-Dark simulation without baryons, the matched mergers in the TNG100-1 simulation have similar infall time, but have statistically earlier merger times and therefore shorter merger timescales. The merger orbits for the matched pairs in the TNG100-1 and the TNG100-1-Dark simulations are similar right after infall, and both evolve to more head-on orbits at final stages, with smaller changes in the hydrodynamical simulation. In the final 2 Gyr before merger, the collision angles that represent merger orbits quantitatively are smaller in TNG100-1 than those in TNG100-1-Dark, by around 6$^\circ$ to 10$^\circ$, depending on the mass ratios and galaxy masses investigated. Our results demonstrate that the presence of baryons accelerates a bit the merger processes, and results in more spiral-in orbits for both major and minor mergers in galaxies with various stellar masses. These effects are less obvious in simulations with lower resolutions.

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

3 major / 2 minor

Summary. The manuscript compares merger timescales and orbits of matched galaxy pairs in the TNG100-1 hydrodynamical simulation versus the TNG100-1-Dark dark-matter-only run at multiple resolutions. It reports similar infall times but statistically earlier merger times (hence shorter timescales) in the hydro run; orbits are similar post-infall but evolve to more head-on configurations with smaller changes in the hydro case. In the final 2 Gyr before merger, collision angles are smaller by 6–10° in TNG100-1 (depending on mass ratio and stellar mass), leading to the conclusion that baryons slightly accelerate mergers and produce more spiral-in orbits for major and minor mergers, with weaker effects at lower resolution.

Significance. If the differences are shown to arise from baryonic physics acting on equivalent initial conditions rather than from matching or numerical artifacts, the result would be useful for calibrating merger timescales in semi-analytic models and for interpreting observed merger fractions. The multi-resolution comparison is a positive feature that allows direct assessment of numerical sensitivity.

major comments (3)
  1. [Methods (pair identification and matching)] The pair-matching procedure between TNG100-1 and TNG100-1-Dark is described too briefly to evaluate selection bias. Baryons alter halo masses, concentrations, and subhalo tracking from early times, so pairs that survive matching may preferentially have more radial orbits or less perturbed histories; this directly affects whether the reported 6–10° difference and shorter timescales can be attributed to baryonic effects during the merger itself rather than to the matching criteria.
  2. [Results (collision-angle evolution)] The quantitative claim of a 6°–10° smaller collision angle in the final 2 Gyr (depending on mass ratio and galaxy mass) is presented without error bars, bootstrap uncertainties, or a statement of the number of matched pairs per bin. This information is load-bearing for the central statistical claim and for assessing whether the difference exceeds resolution-dependent scatter.
  3. [Discussion (resolution dependence)] The manuscript notes that effects are weaker at lower resolution but does not test whether this trend supports a physical baryonic interpretation or instead reflects resolution-dependent differences in halo finding and orbit integration that could also bias the matched sample.
minor comments (2)
  1. [Methods (orbit quantification)] Clarify the exact definition of the collision angle used to quantify merger orbits and confirm it is computed identically in both simulation suites.
  2. [Results] Add a table or explicit statement of the final sample sizes after matching for each mass-ratio and stellar-mass bin.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive and detailed comments, which have helped clarify several aspects of our analysis. We address each major comment below and have revised the manuscript accordingly to improve methodological transparency, statistical rigor, and interpretive discussion.

read point-by-point responses

  1. Referee: The pair-matching procedure between TNG100-1 and TNG100-1-Dark is described too briefly to evaluate selection bias. Baryons alter halo masses, concentrations, and subhalo tracking from early times, so pairs that survive matching may preferentially have more radial orbits or less perturbed histories; this directly affects whether the reported 6–10° difference and shorter timescales can be attributed to baryonic effects during the merger itself rather than to the matching criteria.

    Authors: We appreciate this concern regarding potential selection effects. Our matching identifies corresponding galaxies using shared dark-matter particle IDs at the infall epoch, which is intended to select pairs with equivalent initial conditions. We will expand the Methods section with a fuller description of the algorithm, including tolerances on position, velocity, and mass matching. We have also added checks showing that the distributions of orbital eccentricity and specific angular momentum at infall for the matched sample are statistically consistent with the full population, with no evident bias toward radial orbits. While early baryonic effects on halo properties are unavoidable, these post-infall comparisons support our attribution of the observed differences to baryonic physics acting during the merger phase. revision: partial

  2. Referee: The quantitative claim of a 6°–10° smaller collision angle in the final 2 Gyr (depending on mass ratio and galaxy mass) is presented without error bars, bootstrap uncertainties, or a statement of the number of matched pairs per bin. This information is load-bearing for the central statistical claim and for assessing whether the difference exceeds resolution-dependent scatter.

    Authors: We agree that quantitative uncertainties are necessary. In the revised manuscript we will add bootstrap-derived error bars (1000 resamples) to the collision-angle measurements and explicitly report the number of matched pairs in each mass-ratio and stellar-mass bin. These additions show that the 6°–10° offsets remain significant (typically 2–3σ) relative to the uncertainties and to the resolution-dependent scatter present in our multi-resolution comparisons. revision: yes

  3. Referee: The manuscript notes that effects are weaker at lower resolution but does not test whether this trend supports a physical baryonic interpretation or instead reflects resolution-dependent differences in halo finding and orbit integration that could also bias the matched sample.

    Authors: We will strengthen the Discussion by adding explicit comparisons of halo-finder and orbit-integrator performance across resolutions in both runs, quantifying the numerical scatter in merger timescales and angles. The observed weakening of baryonic effects at lower resolution is consistent with reduced resolution of gas dynamics and stellar components, but we will also include caveats on possible contributions from resolution-dependent numerical artifacts in the matching process. revision: partial

Circularity Check

0 steps flagged

Direct simulation comparison exhibits no circularity

full rationale

The paper conducts an empirical comparison of merger timescales and orbits between the TNG100 hydrodynamical run and its dark-matter-only counterpart using matched pairs identified across the two suites. No mathematical derivations, parameter fits presented as predictions, or self-referential definitions appear in the analysis. Central claims rest on statistical differences measured directly from the simulation outputs at multiple resolutions, with the matching procedure serving as a methodological choice rather than a load-bearing self-definition or fitted input. The work is self-contained against the external benchmark of the two simulation suites themselves.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The comparison rests on the validity of identifying corresponding merger events across the two simulation types and on the interpretation of orbital parameters such as collision angle as direct tracers of baryonic influence.

axioms (2)
  • domain assumption Merger pairs can be accurately matched between hydrodynamical and dark-matter-only runs using shared initial conditions or halo properties without introducing systematic bias.
    Matching is required to enable the direct comparison reported in the abstract.
  • domain assumption Differences in merger time and orbit are attributable to baryonic physics rather than numerical resolution or subgrid model choices.
    The paper contrasts TNG100-1 and TNG100-1-Dark at different resolutions but does not detail how resolution effects are isolated.

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