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arxiv: 2510.03395 · v2 · submitted 2025-10-03 · 🌌 astro-ph.GA

The LMC Corona Favors a First Passage

Scott Lucchini , Jiwon Jesse Han , Sapna Mishra , Andrew J. Fox This is my paper

Pith reviewed 2026-05-18 09:59 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords LMC coronafirst passagesecond passagecircumgalactic mediumtruncation radiusMilky Way interactionorbital trajectorygas stripping
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The pith

Simulations of the LMC's gaseous corona match a first-passage orbit but not a second passage.

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

The paper tests whether the Large Magellanic Cloud is on its first approach to the Milky Way or has completed a prior pericenter passage by modeling the interaction's effect on the LMC's circumgalactic gas. Using simulations with live gas particles inside analytic dark matter potentials that follow published trajectories, only the first-passage case reproduces the observed velocity and column density profiles of the LMC corona. The second-passage case produces excessive stripping over longer interaction time, yielding velocities and densities well below what is seen today. This distinction constrains the recent dynamical history of the LMC-Milky Way system because the current gas properties around the LMC serve as a record of how much time it has spent near the Milky Way.

Core claim

We use constrained idealized simulations of the LMC/Milky Way interaction to determine if the size of the LMC's gaseous halo (Corona) can be used to distinguish between first and second passage models. Using live circumgalactic gas particles combined with analytic dark matter potentials evolved to follow previously published orbital trajectories, we find that the first passage model is able to reproduce the observed velocity profile and column density profile of the present day LMC Corona. On the other hand, in a second passage scenario the longer interaction time leads to the velocities and column densities around the LMC at the present day being significantly lower than observations. Based

What carries the argument

Constrained idealized simulations that combine live circumgalactic gas particles with analytic dark matter potentials evolved along published first- and second-passage orbital trajectories to predict present-day velocity and column density profiles.

If this is right

  • The LMC corona's truncation radius is 16.6 plus or minus 0.5 kpc in the first-passage model, matching observations.
  • The second-passage model yields a truncation radius of only 5.7 plus 1.8 minus 2.2 kpc, inconsistent with data.
  • Longer interaction time in the second-passage case strips gas too efficiently to explain present-day profiles.
  • Gas properties of the LMC's CGM at the present day can be used to rule out second-passage trajectories.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the first-passage preference holds, models of the Magellanic Stream's origin would need to assume a recent single encounter rather than multiple passages.
  • The same live-gas simulation approach could be applied to other Milky Way satellites to infer their orbital histories from CGM observations.
  • Future work incorporating fully live dark matter halos might change the exact truncation radii but is unlikely to reverse the first-versus-second distinction.

Load-bearing premise

The simulations use analytic dark matter potentials and previously published orbital trajectories; if those trajectories or the analytic approximation miss important live-halo or baryonic feedback effects, the predicted truncation and velocity differences would not hold.

What would settle it

A measurement of the LMC corona's truncation radius near 6 kpc together with velocities and column densities significantly below current observations would support the second-passage model instead.

Figures

Figures reproduced from arXiv: 2510.03395 by Andrew J. Fox, Jiwon Jesse Han, Sapna Mishra, Scott Lucchini.

Figure 1
Figure 1. Figure 1: Projected gas density in the first and second passage models at the present day. The top and bottom panels show the MW and LMC CGM gas column density in the Cartesian y −z plane, respectively, with the first passage model on the left and the second passage model on the right. The orbital trajectories of the LMC and MW are drawn in white lines while their present-day positions are marked with plus symbols. … view at source ↗
Figure 2
Figure 2. Figure 2: On-sky projection of the present-day Magellanic Corona and its velocity profile. The top panels show the H II gas column density in Magellanic Coordinates for the first passage model on the left and the second passage model on the right. The white box denotes the region from which the random sightlines were selected to best match the region probed by the observational data in S. Mishra et al. (2024). The b… view at source ↗
Figure 3
Figure 3. Figure 3: LSR velocities and column densities of mock observations of the simulations compared against the data. The top panels show the column density weighted LSR velocities, and the bottom panels show the H II column densities, both as a function of impact parameter from the LMC. The left panels show the results for the first passage model (in red), while the right panels are for the second passage model (in oran… view at source ↗
Figure 4
Figure 4. Figure 4: Distributions of truncation radii (ρT ) for the first and second passage models compared against the range found in S. Mishra et al. (2024) (17−20 kpc, shown in blue). As before, the first passage model is shown in red, and the second passage model is shown in orange. The curves are presented as skewnorm distributions where we have calcu￾lated the root semivariances by finding the range of ρ values where t… view at source ↗
read the original abstract

We use constrained idealized simulations of the LMC/Milky Way interaction to determine if the size of the LMC's gaseous halo (Corona) can be used to distinguish between first and second passage models $-$ an orbital trajectory for the LMC in which it has just recently approached the Milky Way for the first time (first passage), or one in which it has had a previous pericenter (second passage). Using live circumgalactic gas particles combined with analytic dark matter potentials evolved to follow previously published orbital trajectories, we find that the first passage model is able to reproduce the observed velocity profile and column density profile of the present day LMC Corona. On the other hand, in a second passage scenario the longer interaction time leads to the velocities and column densities around the LMC at the present day being significantly lower than observations. Based on this observed velocity profile, recent works have found that the LMC's Corona has been truncated to 17$-$20 kpc, and we find truncation radii of $16.6\pm 0.5$ kpc and $5.7^{+1.8}_{-2.2}$ kpc for the first and second passage models, respectively. Thus, based on the gas properties of the LMC's CGM at the present day, a second passage trajectory is strongly disfavored.

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 / 1 minor

Summary. The paper claims that constrained idealized simulations of the LMC-Milky Way interaction, using live circumgalactic gas particles in analytic dark matter potentials evolved along previously published orbital trajectories, show that a first-passage model reproduces the observed velocity and column density profiles of the LMC Corona with a truncation radius of 16.6 ± 0.5 kpc, consistent with observations of 17-20 kpc. In contrast, a second-passage model results in significantly lower velocities and column densities, yielding a truncation radius of 5.7 kpc, strongly disfavoring the second-passage trajectory based on present-day gas properties.

Significance. If the results hold, this provides a valuable observational constraint on the LMC's orbital history using its CGM properties, with implications for models of the Magellanic system. The quantitative derivation of truncation radii from the simulated profiles and their direct comparison to independent observational estimates is a notable strength, offering a falsifiable test.

major comments (2)
  1. [Methods] The simulations evolve live gas particles inside analytic MW and LMC dark matter potentials whose centers follow fixed, pre-published orbital trajectories. This rigid-potential approximation excludes dynamical friction, live-halo deformation, and gas-induced torques on the relative orbit. As a result, the differential stripping history between first- and second-passage scenarios may not be accurately captured, particularly for the second-passage case which has experienced a prior pericenter; this could affect the reported truncation radii of 16.6±0.5 kpc and 5.7 kpc.
  2. [Results] The truncation radii are reported with uncertainties, but the manuscript does not detail the resolution of the gas particles, the gas cooling implementation, or systematic variations in the analytic potential parameters. These omissions make it difficult to assess the robustness of the factor-of-three difference in truncation radii that underpins the disfavoring of the second-passage model.
minor comments (1)
  1. [Abstract] Consider specifying the exact observational datasets or references for the 'recent works' that report the 17-20 kpc truncation radius.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which help clarify the limitations and presentation of our idealized simulations. We address each major comment below. We agree that additional details on numerical methods are warranted and will incorporate them. On the rigid-potential approximation, we maintain that the controlled comparison still provides a useful constraint on orbital history while acknowledging its simplifications.

read point-by-point responses

  1. Referee: [Methods] The simulations evolve live gas particles inside analytic MW and LMC dark matter potentials whose centers follow fixed, pre-published orbital trajectories. This rigid-potential approximation excludes dynamical friction, live-halo deformation, and gas-induced torques on the relative orbit. As a result, the differential stripping history between first- and second-passage scenarios may not be accurately captured, particularly for the second-passage case which has experienced a prior pericenter; this could affect the reported truncation radii of 16.6±0.5 kpc and 5.7 kpc.

    Authors: We agree that the analytic-potential approach is an approximation that omits dynamical friction, halo deformation, and self-consistent torques. The adopted orbits are taken directly from prior published N-body studies that included these effects, allowing us to isolate the impact of interaction duration on gas stripping. The primary difference between models remains the time elapsed since the most recent pericenter, which drives the contrast in present-day corona properties. We will expand the methods section to explicitly discuss this limitation and its possible influence on the reported truncation radii, while noting that fully live-halo simulations lie beyond the scope of the current controlled study. revision: partial

  2. Referee: [Results] The truncation radii are reported with uncertainties, but the manuscript does not detail the resolution of the gas particles, the gas cooling implementation, or systematic variations in the analytic potential parameters. These omissions make it difficult to assess the robustness of the factor-of-three difference in truncation radii that underpins the disfavoring of the second-passage model.

    Authors: We acknowledge the omission of these technical details. The revised manuscript will include the gas-particle count and mass resolution, the cooling function employed, and the results of parameter-variation tests on the analytic potentials. These additions will demonstrate that the factor-of-three difference in truncation radii is robust within the explored range. We have verified numerical convergence and will report the relevant tests. revision: yes

Circularity Check

0 steps flagged

No significant circularity: forward simulations tested against independent external observations

full rationale

The paper evolves live CGM gas particles inside analytic DM potentials whose centers follow previously published orbital trajectories, then compares the resulting present-day velocity and column-density profiles (and derived truncation radii of 16.6 kpc vs 5.7 kpc) directly to external observational constraints on the LMC Corona (reported truncation 17-20 kpc). These outputs are not obtained by fitting to the target data, nor do they reduce by definition or self-citation to the input orbits; the comparison is falsifiable against independent measurements. No self-definitional, fitted-input, or load-bearing self-citation steps appear in the derivation chain.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The paper rests on standard domain assumptions about ram-pressure stripping and tidal truncation in galaxy interactions plus previously published orbital trajectories; no new free parameters are introduced in the abstract and no new entities are postulated.

axioms (2)
  • domain assumption The LMC corona gas is truncated by ram pressure and tides during close passages with the Milky Way
    This physical mechanism is invoked to explain why longer interaction times in the second-passage case reduce velocities and column densities.
  • domain assumption Analytic dark matter potentials combined with live gas particles sufficiently capture the gravitational and hydrodynamic evolution
    The simulation method described relies on this approximation rather than fully live N-body halos for both galaxies.

pith-pipeline@v0.9.0 · 5776 in / 1509 out tokens · 56481 ms · 2026-05-18T09:59:46.547986+00:00 · methodology

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