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arxiv: 2604.27057 · v1 · submitted 2026-04-29 · 🌌 astro-ph.GA

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Coupling between stellar and HI lopsidedness in Milky Way-type galaxies from the Auriga Superstars cosmological simulations

Arianna Dolfi , Facundo A. G\'omez , Rebekka Bieri , Francesca Fragkoudi , Robert J. J. Grand , Antonela Monachesi , Ruediger Pakmor , Freeke van de Voort
Authors on Pith no claims yet

Pith reviewed 2026-05-07 08:23 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords lopsidednessstellar populationsHI gasgalaxy morphologytidal interactionsgas accretionbar strengthdisk galaxies
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The pith

Old stellar lopsidedness correlates strongly with HI lopsidedness in Milky Way-type galaxies, both tracing global gravitational potential distortions.

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

This paper examines nine Milky Way-like galaxies drawn from cosmological zoom-in simulations to connect asymmetries in stars and atomic hydrogen gas. At the present day, the lopsided shape of stars older than half a billion years matches the lopsided shape of the HI gas, indicating that both components respond to the same large-scale distortions in the galaxy's gravitational potential. Younger stars instead follow patterns set by uneven star formation along spiral arms. Tracking the galaxies backward in time shows that close encounters with massive satellites create lasting lopsidedness across both stars and gas, whereas steady gas infall mainly perturbs the gas and new stars. The results position lopsidedness as a record of a galaxy's interaction and accretion history.

Core claim

The central claim is that morphological lopsidedness measured by the m=1 Fourier mode in old stellar populations (>0.5 Gyr) correlates strongly with the identical measure in the HI gas between 0.5 and 1 optical radii at z=0, showing both trace distortions in the global gravitational potential. Young stars (<0.5 Gyr) instead exhibit lopsidedness tied to asymmetric star formation along spiral arms. Stellar morphological and kinematic lopsidedness correlate tightly, while HI shows weaker morphological-kinematic coupling with kinematics dominating. Bar strength anti-correlates with lopsidedness. Over cosmic time, satellite tidal interactions with mass ratio >1:50 induce coherent lopsidedness in

What carries the argument

The m=1 Fourier mode of face-on mass density and radial velocity maps, applied consistently to stars and HI in the 0.5-1 optical radius range.

Load-bearing premise

The high-resolution cosmological simulations accurately reproduce the dynamical processes that generate lopsidedness in the outer disks without introducing significant numerical artifacts or resolution biases.

What would settle it

A large sample of observed Milky Way-type galaxies in which old stellar morphological lopsidedness fails to correlate with HI lopsidedness at comparable radii would falsify the claimed coupling between the two components.

Figures

Figures reproduced from arXiv: 2604.27057 by Antonela Monachesi, Arianna Dolfi, Facundo A. G\'omez, Francesca Fragkoudi, Freeke van de Voort, Rebekka Bieri, Robert J. J. Grand, Ruediger Pakmor.

Figure 1
Figure 1. Figure 1: Top panels: Present-day face-on V-band stellar surface brightness maps of the nine galaxies in Superstars. The dashed and solid orange circles define the region at 0.5 and 1 Ropt, respectively, where Ropt is calculated as described in Sec. 2.3.1. Bottom panels: Same as the top panels, but showing the present-day face-on HI surface density maps. It is worth noting the presence of a satellite galaxy within 0… view at source ↗
Figure 2
Figure 2. Figure 2: Present-day morphological lopsided profiles of the stellar view at source ↗
Figure 3
Figure 3. Figure 3: Present-day global lopsidedness of the stellar and HI view at source ↗
Figure 4
Figure 4. Figure 4: Top panels: Present-day global lopsidedness of the young stellar component (< 0.5 Gyr old) compared with that of the HI (left) and star-forming (right) gas components. Bot￾tom panel: Same as the top panels for the old stellar component (> 0.5 Gyr old). The symbols have the same meaning as in view at source ↗
Figure 5
Figure 5. Figure 5: Correlation between the present-day global lopsidedness view at source ↗
Figure 6
Figure 6. Figure 6: Face-on projections of the present-day radial velocity maps of the stars and HI disks of the nine galaxies in Superstars. The view at source ↗
Figure 7
Figure 7. Figure 7: Present-day global kinematical lopsidedness compared with the present-day global morphological lopsidedness of the stellar view at source ↗
Figure 8
Figure 8. Figure 8: Global morphological lopsidedness of the stellar ( view at source ↗
read the original abstract

Lopsidedness is common in disk galaxies, yet its origin and evolution remain unclear. Previous studies typically examined stellar and gas asymmetries separately, but a combined analysis offers a stronger probe of the mechanisms driving lopsidedness, recent galaxy evolution, and environment. We analyze the density and kinematics of stellar and atomic hydrogen (HI) components in nine Milky Way type galaxies from the Auriga Superstars cosmological zoom-in simulations. The high stellar mass resolution improves the visibility of disk features while reducing noise, enabling a detailed study of dynamical processes in a cosmological context. Morphological and kinematical lopsidedness are quantified using the first Fourier mode (m=1) of the face-on mass distribution and radial velocity maps, measured consistently for stars and gas between 0.5 and 1 stellar optical radius. At z=0, morphological lopsidedness in old stars (>0.5Gyr) strongly correlates with HI, tracing distortions in the global gravitational potential. In contrast, young stars (<0.5Gyr) trace asymmetric star formation along spiral arms. Stellar morphological and kinematical lopsidedness are strongly correlated, whereas HI shows a weaker correlation, with kinematic asymmetries dominating. We also find an anti-correlation between stellar lopsidedness and bar strength. Strongly barred galaxies tend to host more symmetric disks and higher central stellar mass densities. Tracing lopsidedness evolution over time, tidal interactions with massive satellites (mass ratio >1:50) induce coherent lopsidedness in both stars and HI. In contrast, smooth gas accretion mainly affects HI and young stars, leaving the total stellar component largely symmetric. Overall, these results demonstrate that lopsidedness is a powerful diagnostic of internal disk evolution, gas accretion, and environmental interactions across cosmic time.

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 paper examines morphological and kinematical lopsidedness (m=1 Fourier modes) in the stellar and HI components of nine Milky Way-type galaxies from the Auriga Superstars cosmological zoom-in simulations. Measurements are performed consistently in the 0.5–1 stellar optical radius annulus at z=0 and over time. The central claims are that old stars (>0.5 Gyr) show strong morphological lopsidedness correlation with HI (tracing global potential distortions), while young stars (<0.5 Gyr) trace asymmetric star formation along spirals; stellar morphological and kinematical lopsidedness correlate strongly (HI less so); lopsidedness anti-correlates with bar strength; and tidal interactions induce coherent lopsidedness while smooth accretion primarily affects HI and young stars.

Significance. If the results hold, the work offers a useful combined stellar-population and HI analysis of lopsidedness origins in a cosmological setting, distinguishing internal dynamical responses from environmental drivers. The direct Fourier-mode measurements on high-resolution snapshots and the evolutionary tracking of tidal versus accretion effects are strengths that could inform observational interpretations of disk asymmetries.

major comments (3)
  1. [§2 and §3] §2 (Methods) and §3 (Results): the central claim that old-star morphological lopsidedness traces global potential distortions via correlation with HI rests on the Auriga Superstars runs faithfully capturing m=1 modes in the 0.5–1 R_opt annulus. No convergence tests against lower-resolution Auriga runs, varied softening lengths, or particle-number variations are presented to demonstrate that the reported amplitudes and cross-correlations are stable and free of numerical heating or force inaccuracies that could affect old stars, young stars, and HI differently.
  2. [§3.1] §3.1 (z=0 correlations): the manuscript states that old-star lopsidedness 'strongly correlates' with HI and that stellar morphological/kinematical lopsidedness are 'strongly correlated' while HI shows weaker correlation, yet no correlation coefficients, p-values, bootstrap uncertainties, or sample-variance estimates are provided for the nine-galaxy sample. This absence makes it impossible to assess the statistical robustness of the claimed distinction between old and young stellar responses.
  3. [§3.2] §3.2 (age and radial cuts): the separation into old (>0.5 Gyr) and young (<0.5 Gyr) stars and the fixed 0.5–1 R_opt measurement annulus are used to support the main interpretive claims, but no robustness checks against alternative age thresholds or radial ranges are shown. If the reported correlations weaken or change sign under modest variations, the inference that old stars trace the global potential while young stars trace spiral-arm star formation would be undermined.
minor comments (2)
  1. [Abstract] The abstract would be strengthened by including the sample size (nine galaxies) and at least one quantitative measure of the reported correlations.
  2. [Figures] Figure captions and axis labels should explicitly state the radial annulus (0.5–1 R_opt) and age cuts used for each panel to improve reproducibility.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive and detailed comments, which highlight important aspects for strengthening the statistical and numerical robustness of our analysis. We address each major comment point by point below.

read point-by-point responses

  1. Referee: [§2 and §3] §2 (Methods) and §3 (Results): the central claim that old-star morphological lopsidedness traces global potential distortions via correlation with HI rests on the Auriga Superstars runs faithfully capturing m=1 modes in the 0.5–1 R_opt annulus. No convergence tests against lower-resolution Auriga runs, varied softening lengths, or particle-number variations are presented to demonstrate that the reported amplitudes and cross-correlations are stable and free of numerical heating or force inaccuracies that could affect old stars, young stars, and HI differently.

    Authors: We acknowledge that the manuscript does not include explicit convergence tests for the m=1 Fourier amplitudes and cross-correlations in the 0.5–1 R_opt annulus. The Auriga Superstars simulations adopt a stellar mass resolution of approximately 5×10^3 M_⊙ per particle (roughly an order of magnitude higher than the standard Auriga suite), which was chosen specifically to reduce numerical noise and better resolve disk substructure. Prior Auriga papers have shown convergence of global disk properties at these resolutions. To directly address the concern, we will add a new paragraph and supporting figure in the revised Methods or Results section comparing lopsidedness measurements for galaxies available in both Superstars and standard Auriga runs, confirming that the reported old-star–HI correlations remain stable and are not driven by resolution-dependent effects on different components. revision: yes

  2. Referee: [§3.1] §3.1 (z=0 correlations): the manuscript states that old-star lopsidedness 'strongly correlates' with HI and that stellar morphological/kinematical lopsidedness are 'strongly correlated' while HI shows weaker correlation, yet no correlation coefficients, p-values, bootstrap uncertainties, or sample-variance estimates are provided for the nine-galaxy sample. This absence makes it impossible to assess the statistical robustness of the claimed distinction between old and young stellar responses.

    Authors: The referee correctly identifies the lack of quantitative statistical measures. In the revised manuscript we will report Pearson (and where appropriate Spearman) correlation coefficients together with p-values and bootstrap uncertainties (1000 resamples) for all key relations in §3.1, including old-star vs. HI morphological lopsidedness, stellar morphological vs. kinematical lopsidedness, and the corresponding HI quantities. We will also add a short discussion of sample variance given the limited set of nine galaxies. These additions will allow readers to evaluate the strength and robustness of the claimed distinctions between old and young stellar populations. revision: yes

  3. Referee: [§3.2] §3.2 (age and radial cuts): the separation into old (>0.5 Gyr) and young (<0.5 Gyr) stars and the fixed 0.5–1 R_opt measurement annulus are used to support the main interpretive claims, but no robustness checks against alternative age thresholds or radial ranges are shown. If the reported correlations weaken or change sign under modest variations, the inference that old stars trace the global potential while young stars trace spiral-arm star formation would be undermined.

    Authors: We agree that sensitivity tests to the adopted age threshold and radial annulus are necessary to support the physical interpretation. In the revised §3.2 we will include additional panels or a supplementary table showing the old-star–HI and young-star–HI correlations for alternative age cuts (0.25 Gyr and 1 Gyr) and for shifted radial annuli (0.25–0.75 R_opt and 0.75–1.25 R_opt). We anticipate that the primary trends will persist, because the 0.5 Gyr division separates stars that have undergone significant phase mixing from those formed recently in asymmetric star-forming regions, while the chosen annulus avoids the bar-dominated inner disk. These checks will be presented to confirm the robustness of our conclusions. revision: yes

Circularity Check

0 steps flagged

No significant circularity: direct Fourier measurements on external simulation snapshots

full rationale

The paper's core results are empirical correlations obtained by computing m=1 Fourier amplitudes directly from the face-on mass and radial-velocity fields of the Auriga Superstars snapshots, separately for old stars, young stars, and HI, within the fixed 0.5-1 R_opt annulus. These amplitudes are then cross-correlated across components and with bar strength; no parameter is fitted to a subset of the data and then re-used to 'predict' a closely related quantity, nor is any claimed relation defined in terms of itself. Auriga simulation papers are cited only to identify the input data set; they do not supply a uniqueness theorem or ansatz that forces the reported distinctions. The analysis chain is therefore self-contained post-processing of externally generated snapshots and receives the default non-circularity finding.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on the fidelity of the Auriga cosmological zoom-in simulations and the suitability of the m=1 Fourier mode for quantifying lopsidedness between 0.5 and 1 optical radius.

axioms (1)
  • domain assumption The Auriga Superstars simulations provide a sufficiently accurate representation of Milky Way-type galaxy dynamics and gas physics at the employed resolution.
    Invoked throughout the analysis of density and kinematic maps.

pith-pipeline@v0.9.0 · 5678 in / 1257 out tokens · 67770 ms · 2026-05-07T08:23:47.923491+00:00 · methodology

discussion (0)

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