REVIEW 4 major objections 5 minor 1 cited by
The intricate link between galaxy dynamics and intrinsic shape (or why so-called prolate rotation is a misnomer)
T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read This paper claims that the assumed galaxy shape–rotation relation fails in Illustris, making 'prolate rotation' a misnomer.
desk verdict A clear conference summary of an already-published Illustris result; the plotted trend is plausible, but the paper's strong conclusion about 'little constraining power' is not backed by quantitative analysis. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the intrinsic kinematic misalignment angle $\Psi_{\rm int}$, defined as the angle between the galaxy's angular momentum vector and the major axis of its ellipsoid-equivalent shape, with $\Psi_{\rm int}=90^\circ$ corresponding to rotation around the major axis. The shape itself is measured with an iterative reduced inertia tensor calculation on stellar particles inside the half-mass radius, giving axis ratios $p=b/a$ and $q=c/a$; galaxies are sorted into spherical, oblate, prolate, and triaxial classes from these ratios, and the triaxiality $T=(1-p^2)/(1-q^2)$ is computed. The argument is a direct comparison: the measured $T$–$\Psi_{\rm int}$ scatter in Illustris is checked against the analytic curve that earlier work derived from Stäckel-potential models.
What would settle it
One could falsify the central claim by showing that real IFS galaxies satisfy the $\Psi_{\rm int}$–shape relation: for example, if a large observed sample of galaxies classified as prolate by an independent method shows projected major-axis rotation near $\Psi_{\rm int}\simeq 90^\circ$ as predicted by the analytic relation, the Illustris-based conclusion would fail. A second check would be measuring $\Psi_{\rm int}$ distributions in galaxies from another independent cosmological simulation: if that simulation reproduces the analytic relation, the result is simulation-specific rather than general.
Extended reading notes
Core claim
On the paper's own terms, the discovery is that the analytic relationship between triaxiality $T$ and intrinsic kinematic misalignment $\Psi_{\rm int}$ suggested in earlier theoretical work is not reproduced in Illustris. For oblate and triaxial galaxies the angular momentum vector almost always lies close to the morphological minor axis, giving $\Psi_{\rm int}\simeq 0^\circ$; for prolate and spherical galaxies $\Psi_{\rm int}$ spans the full range from $0^\circ$ to $90^\circ$. In particular, prolate galaxies do not preferentially rotate around their major axis, so the term 'prolate rotation' misdescribes their kinematics. Consequently, $\Psi_{\rm int}$ carries very little information about intrinsic shape, and methods that use this angle to constrain the distribution of galaxy intrinsic shapes are not on solid ground.
Load-bearing premise
The argument depends on Illustris being a faithful stand-in for real galaxies: if the simulated population's angular momentum content or shape distribution differs from the observed Universe, the conclusion that observational shape-recovery is biased does not necessarily follow.
Editorial extensions
If this is right
- Published intrinsic shape distributions inferred from IFS surveys such as ATLAS3D, SAMI, MANGA, and MASSIVE that rely on the $\Psi_{\rm int}$–shape relation may be systematically biased, because a key prior in the inference is invalid.
- Kinematic maps alone cannot break the shape degeneracy: $\Psi_{\rm int}$ should be treated as a weak or uninformative constraint rather than a sharp predictor.
- Prolate galaxies should not be assumed to rotate around their projected major axis; classification schemes that equate 'prolate rotation' with a particular $\Psi_{\rm int}$ value need revision.
- Forward-modeling simulated galaxies through observational selection effects would be a safer route to infer intrinsic shape distributions than the analytic relation.
Reading between the lines
- If the Illustris result is representative, earlier observational shape-recovery results that used the $\Psi_{\rm int}$ relation likely underestimate the fraction of misaligned prolate galaxies; re-analyzing existing IFS data with simulation-calibrated priors would test this.
- The same test could be run in other independent cosmological simulations to see whether the breakdown of the $\Psi_{\rm int}$–shape relation is a generic prediction of galaxy formation physics or specific to Illustris.
- A useful observable extension would be to compare the projected distribution of kinematic misalignment angles predicted by simulating Illustris snapshots with observed IFS samples; a mismatch would help refine the feedback or angular momentum implementations.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper uses 978 galaxies from the Illustris cosmological simulation at z=0 to measure intrinsic ellipsoidal axis ratios (p, q) and the intrinsic kinematic misalignment angle Psi_int between the stellar rotation vector and the morphological major axis. The authors compare the joint distribution of triaxiality and Psi_int with the analytical relation suggested by Weijmans et al. (2014). They report that most oblate and triaxial Illustris galaxies have Psi_int near 0 degrees, while prolate and spherical galaxies scatter broadly in Psi_int, with no preference for Psi_int near 90 degrees. From this they conclude that Psi_int holds little constraining power for recovering galaxy intrinsic shape distributions, and that the term 'prolate rotation' is a misnomer.
Significance. The question addressed is timely and important: several large IFS surveys (ATLAS3D, SAMI, MANGA, MASSIVE) have used the assumed relationship between kinematics and intrinsic shape to statistically recover shape distributions, so a demonstration that this relationship fails would have direct practical impact. The paper's strengths are that it uses a large, well-defined simulated sample with a clear stellar-particle selection threshold, an iterative reduced-inertia-tensor shape measurement, and a direct visual comparison to an external analytical relation. However, as written the paper provides only a scatter plot as evidence; there is no quantitative test of constraining power, no uncertainty estimate, and no validation of the simulation against observed misalignment distributions. If the central claim were supported by quantitative inference tests, the result would be significant for the interpretation of IFS shape-recovery studies.
major comments (4)
- [§3, Fig. 1 (right panel)] The conclusion that 'Psi_int holds little constraining power when inferring the distribution of galaxy intrinsic shapes' is not established by the scatter plot alone. A variable with a noisy or non-monotonic joint distribution can still be informative; for example, the strong clustering of oblate and triaxial galaxies at Psi_int near 0 degrees is itself a constraint that separates them from prolate and spherical galaxies. The paper should provide a quantitative measure of constraining power, such as mutual information, classification accuracy, or a likelihood-based comparison of P(shape | Psi_int), or a mock shape-recovery experiment that measures how well the true shape distribution is recovered with and without the assumed relation.
- [§3] The statement that the failure of the Weijmans et al. (2014) relation 'affects our ability to recover accurate intrinsic shape distributions' is an inference that goes beyond the figure. Standard forward-modeling methods, such as those used by Foster et al. (2017), marginalize over the full joint distribution of intrinsic shape and misalignment; a broken one-to-one relation does not automatically imply a biased recovered shape distribution. The authors should demonstrate the claimed bias explicitly, for instance by applying a shape-recovery method to simulated galaxies with known intrinsic shapes and comparing the inferred distribution with and without the assumed relation.
- [§2.2, Eq. (2.4)] The text states that Psi_int is 'the angle between the short axis of the equivalent ellipsoid and the stellar angular rotation vector', but Eq. (2.4) defines it using the angle between the rotation vector and the major axis e1. This is a direct inconsistency in the definition of the central quantity of the paper. Please reconcile the text and equation, and also explain how the major axis is determined robustly for spherical galaxies, whose major-axis direction is stochastic, since those galaxies contribute to the scatter shown in Figure 1.
- [§2–§3] The paper draws observational implications from Illustris without validating that the simulation reproduces the observed distribution of kinematic misalignments. The conclusion that real shape-recovery methods are biased depends on Illustris being a faithful proxy for galaxy angular-momentum content and shapes, especially at the low-mass, prolate end. Please include a comparison of the projected kinematic misalignment distribution of the simulated sample with observed IFS samples (e.g., ATLAS3D, SAMI, or MANGA), or otherwise justify that the simulated dynamics are representative, before generalizing to observational surveys.
minor comments (5)
- [Abstract] The phrase 'Many recent integral integral field spectroscopy' contains a duplicated word 'integral'; please correct.
- [Eq. (2.3)] The notation 'Ln ⃗ vn' appears to be a typesetting error; it should likely be the luminosity L_n times the velocity vector \vec{v}_n. Please clarify the vector notation in this equation.
- [Fig. 1] The right-hand panel would benefit from error bars or at least a statement of the typical measurement uncertainty on Psi_int and triaxiality, since the visual scatter is compared qualitatively with the Weijmans et al. relation.
- [References] The reference to Mendez-Abreu (2016) is malformed ('ASSL, 15, ASSL..418'); please provide the full bibliographic entry.
- [§3] The functional form of the Weijmans et al. (2014) relation that is drawn as a red dashed line is not specified in the text; please state the equation or describe the relation quantitatively so that the comparison in Figure 1 is reproducible.
Circularity Check
No circularity: the paper is an empirical comparison against an external analytical relation, with only a non-load-bearing self-citation.
full rationale
This is an empirical comparison, not a derivation. The paper measures p, q, T, and Ψ_int directly from Illustris simulation particles using Eqs. (2.1)–(2.4), then compares these measurements to the relationship from Weijmans et al. (2014), which is an external, independent source based on Stäckel-potential models. No parameter is fitted to the target conclusion; the conclusion that prolate galaxies scatter in Ψ_int follows from the plotted joint distribution. The definition of Ψ_int relative to the major axis is a stated modeling choice (Section 2.2), not an input that encodes the result. The only self-reference is to Bassett & Foster (2019), the original paper containing the same Illustris analysis; that citation is provenance for this proceedings summary and is not load-bearing in the argument. The claim that Ψ_int holds 'little constraining power' is an extrapolation from a scatter plot and may be under-supported—that is a correctness or inference-strength concern, not circularity.
Assumptions & free parameters
free parameters (1)
- Shape classification thresholds =
p-q = 0.2; p = 0.8
assumptions (4)
- domain assumption Illustris simulations approximate real galaxy dynamics closely enough to draw conclusions about observed galaxies.
- domain assumption Galaxy stellar distributions can be approximated by a single ellipsoid-equivalent shape.
- domain assumption The Weijmans et al. (2014) relation, derived for Stäckel-potential elliptical galaxies, is the appropriate baseline for IFS galaxy samples.
- standard math Eigenvalue decomposition of the reduced inertia tensor recovers the intrinsic axes.
Cite this review
Pith. "Pith review of The intricate link between galaxy dynamics and intrinsic shape (or why so-called prolate rotation is a misnomer)." pith.science (2026). https://pith.science/paper/6J7ZKM3M
@misc{pith2026190808648,
author = {Pith},
title = {Pith review of: The intricate link between galaxy dynamics and intrinsic shape (or why so-called prolate rotation is a misnomer)},
year = {2026},
howpublished = {\url{https://pith.science/paper/6J7ZKM3M}},
note = {Machine review of arXiv:1908.08648}
}
read the original abstract
Many recent integral integral field spectroscopy (IFS) survey teams have used stellar kinematic maps combined with imaging to statistically infer the underlying distributions of galaxy intrinsic shapes. With now several IFS samples at our disposal, the method, which was originally proposed by M. Franx and collaborators in 1991, is gaining in popularity, having been so far applied to ATLAS3D, SAMI, MANGA and MASSIVE. We present results showing that a commonly assumed relationship between dynamical and intrinsic shape alignment does not hold in Illustris, affecting our ability to recover accurate intrinsic shape distributions. A further implication is that so-called "prolate rotation", where the bulk of stars in prolate galaxies are thought to rotate around the projected major axis, is a misnomer.
Figures
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