Mapping the Distorted Dark Matter Distribution of the LMC-SMC System Prior to Milky Way Infall with Basis Function Expansions

Chervin F. P. Laporte; Ekta Patel; Facundo A. G\'omez; Gurtina Besla; Hayden R. Foote; Himansh Rathore; Martin D. Weinberg; Michael S. Petersen; Nicol\'as Garavito-Camargo

arxiv: 2601.00946 · v2 · submitted 2026-01-02 · 🌌 astro-ph.GA

Mapping the Distorted Dark Matter Distribution of the LMC-SMC System Prior to Milky Way Infall with Basis Function Expansions

Hayden R. Foote , Himansh Rathore , Gurtina Besla , Nicol\'as Garavito-Camargo , Ekta Patel , Michael S. Petersen , Martin D. Weinberg , Facundo A. G\'omez

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Chervin F. P. Laporte

This is my paper

Pith reviewed 2026-05-16 17:33 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords LMCSMCdark matter halosdynamical frictionN-body simulationbasis function expansionsMilky Way infallsatellite interactions

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The pith

The LMC-SMC 1:10 encounter creates a 20 kpc wake and dual overdensities in the LMC dark matter halo at the time of Milky Way infall.

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

The paper runs a high-resolution N-body simulation of the Large and Small Magellanic Clouds interacting in isolation and applies basis function expansions to map their dark matter distributions at an epoch chosen to represent the moment before they fall into the Milky Way. The smaller SMC induces a 20 kpc dynamical friction wake in the LMC halo and repeatedly displaces the LMC density center, leaving two overdensities at roughly 60 and 100 kpc. In response the SMC loses two-thirds of its dark matter and shrinks to a 4 kpc tidal radius. The resulting gravitational field is strongly asymmetric and evolves with time, so any accurate orbit integration for the Clouds must use the changing halo shapes rather than fixed approximations. These mutual distortions mirror those expected in the Milky Way-LMC encounter and appear across host-mass scales.

Core claim

Using basis function expansions of a high-resolution N-body simulation of the LMC-SMC system in isolation at the epoch of Milky Way infall, the LMC halo develops a 20 kpc dynamical friction wake and two overdensities at approximately 60 and 100 kpc produced by repeated SMC-induced displacements of the LMC density center; the SMC simultaneously loses two-thirds of its initial dark matter mass and reaches a tidal radius of only 4 kpc, generating a highly asymmetric and time-dependent acceleration field throughout the system.

What carries the argument

Basis function expansions constructed from the particle data of the N-body simulation, which decompose the time-evolving, non-spherical density and gravitational potential of each halo without assuming spherical symmetry.

If this is right

Orbit calculations for the LMC-SMC system must use time-dependent, asymmetric halo shapes instead of static spherical potentials.
The LMC halo distortions already present at infall will alter the subsequent interaction with the Milky Way dark matter halo.
1:10 mass-ratio satellite-host encounters produce recognizable wakes and overdensities in both halos across a range of host masses.
Merger rate estimates and tests of dark matter models must incorporate these pre-infall halo deformations.

Where Pith is reading between the lines

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

Stellar streams or satellite distributions around the LMC could carry observable signatures of the pre-infall wake and overdensities.
Extending the basis function method to simulations that include the Milky Way potential would show how external gravity modifies the internal distortions.
Similar halo deformations in other galaxy groups would bias merger-rate inferences if static halo models are assumed.

Load-bearing premise

The isolated LMC-SMC simulation at the chosen infall epoch accurately captures the halo distortions that would form when the Milky Way's gravity and full prior orbital history are also included.

What would settle it

Direct or indirect mapping of the LMC dark matter distribution at infall showing neither a 20 kpc wake nor overdensities at 60 and 100 kpc would falsify the predicted characteristic deformations.

read the original abstract

The SMC orbits within the LMC's dark matter (DM) halo in a $\sim$1:10 mass-ratio encounter. The LMC:Milky Way (MW) interaction is also $\sim$1:10, and is expected to perturb the MW's DM distribution. However, no framework exists to quantify the severity of these perturbations over multiple pericenters and longer periods of time, such as the LMC-SMC interaction history. We construct basis function expansions of a high-resolution \textit{N}-body simulation of the Clouds interacting in isolation and analyze their DM distributions at an epoch approximating the time of their infall to the MW. Our goal is to quantify how the Clouds distort each other's DM distributions \textit{without} the MW. The LMC halo's response to the SMC includes a $\sim 20$ kpc long dynamical friction wake and the displacement of the LMC's density center during each SMC pericenter, which produces two overdensities in the LMC halo (at $\sim$60 and $\sim$100 kpc) at MW infall. The SMC's tidal radius at infall is just $\sim4$ kpc, at which point the SMC has lost two-thirds of its initial DM mass to the LMC. The distortions to the Clouds' halos produce a highly asymmetric acceleration field. Accurate orbit integration in the LMC-SMC system must account for the time-dependent shapes of both halos. The SMC-induced perturbations in the LMC DM halo resemble the MW-LMC system, and persist over multiple SMC pericenters. We conclude that 1:10 satellite-host encounters induce characteristic deformations in both DM halos across host-mass scales, with implications for merger rates and tests of DM models.

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.

Desk Editor's Note private letter to a colleague

The paper maps concrete DM distortions from an isolated LMC-SMC N-body run at infall using basis expansions, but the lack of Milky Way tidal field limits how directly the numbers apply to the real system.

read the letter

The paper runs a high-resolution N-body simulation of the LMC and SMC in isolation up to an epoch meant to match their infall into the Milky Way, then applies basis function expansions to track the dark matter responses. It reports a roughly 20 kpc dynamical friction wake in the LMC halo, two overdensities at 60 and 100 kpc from displaced density centers, an SMC tidal radius of only 4 kpc, and two-thirds loss of the SMC's initial dark matter mass. The resulting acceleration field is strongly asymmetric, and the authors note that orbit integration needs to account for these time-dependent halo shapes. The SMC-induced features in the LMC halo look similar to what is expected in the MW-LMC case and survive multiple pericenters. This supplies specific, quantitative templates for 1:10 encounters that were not in the earlier literature cited in the abstract. The BFE approach is a reasonable way to extract the wakes and overdensities cleanly from the snapshot, and the numbers are presented as direct outputs from the run rather than from heavy fitting. That part of the work is straightforward and could be checked if the initial conditions and resolution details are fully documented. The central limitation is the isolation assumption. The real Clouds have spent time in the Milky Way potential, so earlier tidal stripping and orbital history would alter pericenter distances, mass loss rates, and halo responses. The reported wake length and overdensity locations might shift once the MW field is included, and the paper does not test that addition. The chosen infall epoch is also an approximation whose sensitivity is not explored in the abstract. This is useful for modelers working on local-group dynamics or Milky Way mass estimates who need a starting point for distorted halo shapes. It is a competent simulation study with clear implications, so it deserves peer review even though reviewers will need to press on the isolation choice and whether the quantitative features hold up under more complete potentials.

Referee Report

1 major / 2 minor

Summary. The paper analyzes a high-resolution N-body simulation of the LMC-SMC system evolved in isolation up to an epoch approximating MW infall. Using basis function expansions, it maps the DM halo distortions, reporting a ~20 kpc dynamical friction wake in the LMC, displaced density centers producing overdensities at ~60 and ~100 kpc, ~2/3 DM mass loss from the SMC (whose tidal radius is ~4 kpc), and a highly asymmetric acceleration field. The central claim is that these features are characteristic of 1:10 satellite-host encounters across mass scales, with implications for merger rates and DM model tests; the authors note the simulation omits the MW potential.

Significance. If the reported deformations hold under more complete modeling, the work supplies a concrete BFE-based framework for quantifying time-dependent halo responses in 1:10 encounters, directly addressing the lack of such tools noted in the abstract. The high-resolution isolated run and explicit mass-loss and wake measurements provide falsifiable predictions that could be tested against observations or full MW-inclusive simulations, strengthening constraints on satellite dynamics and DM halo shapes.

major comments (1)

[Simulation setup and conclusions] The central claim that the observed ~20 kpc wake, displaced overdensities, and 2/3 mass loss are characteristic of the real LMC-SMC system at infall (and of 1:10 encounters generally) rests on the isolated simulation. However, the setup omits the MW tidal field and the Clouds' prior orbital history, which the skeptic note correctly identifies as potentially altering pericenter distances, tidal radii, and halo responses; this assumption is load-bearing for extrapolating the reported features to the actual infall epoch.

minor comments (2)

[Methods] Clarify the precise criteria used to select the 'infall epoch' snapshot and quantify how sensitive the reported wake length and overdensity positions are to small shifts in that choice.
[Results] The abstract states the SMC has lost two-thirds of its initial DM mass; provide the corresponding initial and final bound-mass values and the algorithm used to identify bound particles.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading and constructive feedback. We address the major comment on the simulation setup below, agreeing that the omission of the MW potential is a significant caveat while defending the value of the isolated baseline analysis.

read point-by-point responses

Referee: [Simulation setup and conclusions] The central claim that the observed ~20 kpc wake, displaced overdensities, and 2/3 mass loss are characteristic of the real LMC-SMC system at infall (and of 1:10 encounters generally) rests on the isolated simulation. However, the setup omits the MW tidal field and the Clouds' prior orbital history, which the skeptic note correctly identifies as potentially altering pericenter distances, tidal radii, and halo responses; this assumption is load-bearing for extrapolating the reported features to the actual infall epoch.

Authors: We agree that the isolated setup omits the MW tidal field and the Clouds' full prior orbital history, which limits direct extrapolation to the real infall epoch and represents a genuine caveat for the strongest version of our central claim. Our simulation was deliberately constructed in isolation to isolate and quantify the DM distortions arising solely from the LMC-SMC 1:10 encounter, providing a necessary baseline before adding the MW. The reported wake, overdensities, and mass loss are direct consequences of this encounter and persist over multiple pericenters in the run. We will revise the manuscript to (i) more prominently emphasize that these features are measured in the absence of the MW, (ii) add a dedicated discussion subsection on how the MW potential could modulate pericenter distances, tidal radii, and halo responses, and (iii) temper the language around 'characteristic of the real LMC-SMC system' to 'characteristic of isolated 1:10 encounters, with implications for the real system pending MW-inclusive modeling.' This addresses the load-bearing assumption without altering the core results or requiring new simulations. We view this as a partial revision. revision: partial

Circularity Check

0 steps flagged

No circularity; results are direct outputs of isolated N-body simulation analysis

full rationale

The paper's derivation consists of running a high-resolution N-body simulation of the LMC-SMC system in isolation, constructing basis function expansions of the DM distributions, and analyzing snapshot features (wake, overdensities, mass loss, asymmetric field) at a chosen epoch approximating MW infall. These quantities are computed directly from the simulation particles and BFE coefficients rather than from any equation that reduces to a fitted parameter or prior self-citation. The generalization to 'characteristic deformations in 1:10 encounters' is an interpretive statement based on the single simulated case, not a mathematical prediction forced by the inputs. No self-definitional steps, fitted-input predictions, or load-bearing self-citations appear in the provided text; the central mapping remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the assumption that the isolated N-body run plus BFE analysis faithfully represents the pre-infall state; no free parameters are explicitly fitted in the abstract, but the choice of simulation initial conditions and the epoch selected for analysis function as implicit assumptions.

axioms (2)

domain assumption The LMC-SMC interaction history can be approximated by an isolated high-resolution N-body simulation without the Milky Way potential.
Stated in the abstract as the setup used to quantify distortions without the MW.
domain assumption Basis function expansions accurately capture the time-dependent, non-spherical dark matter distributions at the chosen snapshot.
The method is invoked to map the halos; its validity for highly distorted configurations is taken as given.

pith-pipeline@v0.9.0 · 5683 in / 1498 out tokens · 50270 ms · 2026-05-16T17:33:28.388673+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

We construct basis function expansions of a high-resolution N-body simulation of the Clouds interacting in isolation... The LMC halo’s response to the SMC includes a ∼20 kpc long dynamical friction wake...
IndisputableMonolith/Foundation/ArithmeticFromLogic.lean embed_injective unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

The SMC’s tidal radius at infall is just ∼4 kpc, at which point the SMC has lost two-thirds of its initial DM mass to the LMC.

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extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
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