arxiv: 2603.21279 · v2 · submitted 2026-03-22 · 🌌 astro-ph.GA

Revisiting the Excess of Bar-like Structures in TNG50 Early-type Galaxies: Consistency and Tension with Observations

Hangci Du , Yougang Wang , Junqiang Ge This is my paper

Pith reviewed 2026-05-15 07:06 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords galaxy barsearly-type galaxiesTNG50 simulationpattern speedsgalaxy quenchingdispersion-dominated galaxiesbar evolutiondynamical friction

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The pith

Bar-like structures in TNG50 early-type galaxies are slow, long-lived fossils that began as fast bars in gas-rich disks at higher redshifts and survived quenching.

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

The paper uses a morphology-agnostic census to examine elongated features in TNG50 early-type galaxies at z=0. These structures appear in 75-80 percent of dispersion-dominated systems, showing coherent but slow pattern speeds rather than acting as prolate rotators. Tracing their histories shows they form as ordinary fast bars in gas-richer disks at z greater than or equal to 0.2, then decelerate and lengthen through dynamical friction as the galaxies quench into red, gas-poor states. The excess of such bars in the simulation likely stems from imperfect baryonic physics that overproduces long-lived bars or from an observational bias against detecting secularly evolved bars in hot stellar systems.

Core claim

In TNG50, the bar-like features inside early-type galaxies are genuine non-axisymmetric instabilities with lengths of at least 3 kpc and pattern speeds below 20 km s^{-1} kpc^{-1}. They sit in red, gas-poor, dispersion-supported hosts and evolve from typical fast bars present in gas-richer disks at earlier epochs, persisting through the quenching phase via secular deceleration and lengthening driven by dynamical friction.

What carries the argument

Secular deceleration and lengthening of bar pattern speeds through dynamical friction during galaxy quenching.

If this is right

These structures persist as fossil bars in the red sequence after galaxies quench.
Fast bars in late-type galaxies can transition into slow bars inside early-type galaxies.
The simulation overproduces such bars relative to current observations or observations miss long-lived bars in hot systems.
Bar evolution continues through the quenching phase rather than stopping at the transition to early-type morphology.

Where Pith is reading between the lines

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

If correct, galaxy evolution models must include bar survival mechanisms inside dispersion-dominated hosts.
Re-examination of observed early-type galaxies with deeper imaging or kinematic data could reveal a hidden population of slow bars.
The same deceleration process may operate in real galaxies, offering a way to test simulation quenching prescriptions against bar kinematics.
Connecting bar properties across redshift could tighten constraints on when and how disks lose gas and angular momentum.

Load-bearing premise

TNG50 baryonic physics produces realistic bar evolution and quenching without creating an artificial excess of long-lived bars, and the morphology-agnostic method correctly flags genuine bars rather than simulation artifacts.

What would settle it

Measurements of bar pattern speeds and lengths in a large sample of observed early-type galaxies that show no population of slow, long bars in red, gas-poor, dispersion-dominated systems.

read the original abstract

The IllustrisTNG simulation suite, particularly TNG50, was reported to have generated a notable population of elongated, bar-like structures within galaxies classified as Early-Type Galaxies (ETGs). In this work, we revisit the nature of these structures at $z=0$ using a morphology-agnostic census. We find that these features are ubiquitous ($f_{\rm bar} \sim 75-80\%$) in dispersion-dominated galaxies ($D/T < 0.2$) in TNG50-1. They are not prolate rotators (rotating around their long axis), but genuine non-axisymmetric instabilities characterized by coherent, albeit slow, pattern speeds. Unlike the fast bars found in Late-Type Galaxies, these bar-like structures in ETGs are physically longer ($\gtrsim 3$ kpc), rotate significantly slower ($\Omega_p \lesssim 20$ km s$^{-1}$ kpc$^{-1}$), and reside in red, gas-poor, dispersion-dominated systems. By tracing the evolutionary history of these systems, we demonstrate that such structures originate as typical fast bars in gas-richer discs at higher redshifts ($z \gtrsim 0.2$). They survive the galaxy quenching phase, undergoing secular deceleration and lengthening due to dynamical friction, ultimately appearing as slow, fossilized rotators in the $z=0$ red sequence. We conclude that the specific excess of bar-like structures in TNG50 ETGs likely reflects a combination of the imperfect baryonic physics of the simulation (over-producing these bar-like structures or their host ETGs) and a potential observational blind spot regarding long-lived, secularly evolved bars in hot stellar systems.

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

TNG50 ETGs host slow, long bars that evolve from fast bars at z>0.2 and survive quenching, but the excess likely traces back to simulation physics rather than new observational insight.

read the letter

The main thing to know is that the paper traces bar evolution in TNG50-1 galaxies from z~0.2 to z=0. These structures begin as typical fast bars in gas-richer disks, then lengthen and slow to pattern speeds below 20 km/s/kpc as the hosts quench into dispersion-dominated systems with D/T below 0.2. At z=0 they sit in red, gas-poor ETGs and measure longer than 3 kpc. The analysis uses direct measurements of coherent pattern speeds to rule out prolate rotators and shows the bars persist through secular deceleration and dynamical friction. This evolutionary path is the clearest new piece; prior work noted the high bar fraction in simulated ETGs, but the time-tracing adds the origin story and links it to quenching. Definitions are explicit and the internal measurements on the public TNG50 data look consistent. The paper does a clean job separating the simulation output from external observations and avoids circular definitions. The soft spot is the attribution of the excess to imperfect baryonic physics or an observational blind spot. The comparison to real ETG bar fractions does not re-apply the same selection cuts or completeness estimates inside the simulation volume, so the tension remains partly unquantified. There are also no physics-variation runs or cross-checks against other simulations to test whether the long-lived slow bars are physical or an artifact of TNG50's AGN feedback and stellar heating. The stress-test concern about possible numerical stabilization in low D/T systems is not directly tested here. This work is for researchers who model bar dynamics and quenching in cosmological simulations. It supplies a concrete evolutionary sequence that anyone running similar analyses can use as a benchmark. It deserves peer review because the tracing is reproducible from the public data and the tension it highlights is worth referee scrutiny, even if the root-cause claim needs more support.

Referee Report

2 major / 2 minor

Summary. The manuscript analyzes bar-like structures in TNG50-1 early-type galaxies (ETGs) at z=0 via a morphology-agnostic census. It reports that these features occur in 75-80% of dispersion-dominated systems (D/T < 0.2), are genuine non-axisymmetric instabilities with coherent slow pattern speeds (Ω_p ≲ 20 km s^{-1} kpc^{-1}) and lengths ≳ 3 kpc, and reside in red, gas-poor hosts. Evolutionary tracing shows they form as typical fast bars in gas-richer disks at z ≳ 0.2, survive quenching, and secularly decelerate and lengthen into fossilized slow rotators. The excess relative to observations is attributed to imperfect baryonic physics in the simulation or an observational blind spot for long-lived bars in hot systems.

Significance. If the traced evolutionary path and pattern-speed measurements hold, the work supplies a physically motivated resolution to the reported excess of bars in simulated ETGs, connecting bar evolution directly to quenching and dynamical friction. It provides concrete, falsifiable predictions for the lengths and speeds of bars in red-sequence galaxies and highlights the sensitivity of bar demographics to subgrid feedback prescriptions. The direct, simulation-internal measurements of Ω_p and morphological parameters constitute a strength.

major comments (2)

[Evolutionary history and conclusion] The central conclusion that the excess arises from 'imperfect baryonic physics' (abstract and final section) is load-bearing yet rests on the untested assumption that TNG50's AGN feedback and stellar heating do not artificially stabilize long-lived bars in dispersion-dominated systems. No physics-variation runs or cross-simulation comparisons (e.g., with EAGLE) are presented to isolate this effect.
[Bar identification and pattern-speed measurement] In the bar census for D/T < 0.2 galaxies, the claim that the structures are supported by x1 orbits rather than numerical artifacts requires explicit demonstration that the measured Ω_p ≲ 20 km s^{-1} kpc^{-1} remains stable under changes in force softening or particle number; the current morphology-agnostic identification does not include such a robustness test.

minor comments (2)

[Sample selection] The exact numerical threshold and definition of the D/T ratio used to select the ETG sample should be stated explicitly in the methods section rather than referenced only by citation.
[Figures] Figure captions for the evolutionary tracks should include the number of galaxies in each redshift bin to allow assessment of statistical robustness.

Simulated Author's Rebuttal

2 responses · 2 unresolved

We thank the referee for their constructive comments and for recognizing the significance of our evolutionary tracing and pattern-speed measurements. We respond point-by-point to the major comments below.

read point-by-point responses

Referee: The central conclusion that the excess arises from 'imperfect baryonic physics' (abstract and final section) is load-bearing yet rests on the untested assumption that TNG50's AGN feedback and stellar heating do not artificially stabilize long-lived bars in dispersion-dominated systems. No physics-variation runs or cross-simulation comparisons (e.g., with EAGLE) are presented to isolate this effect.

Authors: We agree that the conclusion relies on the evolutionary path traced within TNG50 and that direct isolation of AGN feedback or stellar heating effects would require physics-variation runs or cross-code comparisons, which are absent here. The tracing demonstrates that the structures form as fast bars at z ≳ 0.2 and decelerate secularly, but this does not rule out simulation-specific stabilization. In revision we will soften the language in the abstract and final section to 'may reflect a combination of imperfect baryonic physics or an observational blind spot' and add a dedicated paragraph calling for future comparisons with EAGLE or TNG feedback variations to test the robustness of the bar excess. revision: yes
Referee: In the bar census for D/T < 0.2 galaxies, the claim that the structures are supported by x1 orbits rather than numerical artifacts requires explicit demonstration that the measured Ω_p ≲ 20 km s^{-1} kpc^{-1} remains stable under changes in force softening or particle number; the current morphology-agnostic identification does not include such a robustness test.

Authors: The manuscript identifies the structures via a morphology-agnostic Fourier method showing coherent pattern speeds and does not explicitly invoke x1 orbits. We acknowledge that no dedicated convergence tests varying softening length or particle number were performed. The coherence of Ω_p across radii and its consistency with the traced secular evolution provide indirect support against artifacts, and TNG50 resolution is standard for such analyses. In revision we will expand the methods section with a discussion of the adopted softening and particle masses together with references to convergence studies in the literature, while noting that a full resolution-variation test lies beyond the present scope. revision: partial

standing simulated objections not resolved

No physics-variation runs or cross-simulation comparisons (e.g., with EAGLE) are available to isolate baryonic physics effects.
No explicit robustness test of Ω_p stability under changes in force softening or particle number is provided.

Circularity Check

0 steps flagged

Direct simulation measurements with no load-bearing circularity

full rationale

The paper's central results derive from direct measurement of pattern speeds, lengths, and evolutionary tracing within the TNG50 simulation snapshots. No quantity is defined in terms of itself, no fitted parameter is relabeled as a prediction, and no uniqueness theorem or ansatz is imported via self-citation to force the outcome. The evolutionary origin claim follows from following the same galaxies across redshift in the simulation data. Any self-citations are peripheral and do not carry the main argument.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claims rest on the fidelity of TNG50 baryonic physics for bar dynamics and on the assumption that pattern-speed coherence reliably identifies genuine bars rather than numerical artifacts.

free parameters (1)

D/T threshold
Dispersion-dominated galaxies defined by D/T < 0.2; this cut selects the sample in which bars are reported as ubiquitous.

axioms (1)

domain assumption TNG50 baryonic physics produces realistic bar formation and secular evolution
Invoked when interpreting the evolutionary history and when attributing excess bars to imperfect physics rather than simulation error.

pith-pipeline@v0.9.0 · 5620 in / 1384 out tokens · 58246 ms · 2026-05-15T07:06:00.771498+00:00 · methodology

discussion (0)

Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

The Local Tremaine-Weinberg Method for Galactic Pattern Speed: Theory and its Application to IllustrisTNG
astro-ph.GA 2026-03 unverdicted novelty 7.0

A local Tremaine-Weinberg framework integrates the continuity equation over flexible loops to measure galactic pattern speeds, recovering standard methods as special cases and validated on IllustrisTNG simulations.