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arxiv: 2604.01758 · v1 · submitted 2026-04-02 · 🌌 astro-ph.GA

Diverse lifestyles of bar-like galaxies and their coevolution with the brightest galaxy in the most massive cluster of TNG50

Ewa L. Lokas This is my paper

Pith reviewed 2026-05-13 21:16 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords bar-like galaxiesbrightest cluster galaxyTNG50 simulationgalaxy clusterstidal interactionsgalaxy evolutionpericenter passages
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The pith

Eleven bar-like galaxies form or strengthen their bars during pericenter passages around progenitors of the brightest cluster galaxy.

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

The paper examines 15 galaxies with strongly prolate stellar components in the most massive cluster from the TNG50 simulation. Eleven of these galaxies form or enhance bars during close approaches to one or more early versions of the brightest cluster galaxy. The remaining galaxies develop bars through minor mergers, other galaxy interactions, or internal disk processes. Bar formation occurs over a wide time range of 3 to 11 billion years ago, with bar lengths between 2 and 6 kiloparsecs that show no link to the strength of tidal forces. All selected galaxies experience at least one such passage, though the total number of interactions varies and correlates with different levels of mass loss.

Core claim

In the most massive cluster of the TNG50 simulation, 15 galaxies are selected for having strongly prolate stellar components. Eleven form or strongly enhance bars during a pericenter passage around one or more progenitors of the brightest cluster galaxy. Two form bars early via minor mergers, one through interaction with another massive galaxy, and one via disk instability. Formation times range from 3 to 11 Gyr ago and bar lengths from 2 to 6 kpc with no correlation to tidal forcing. Most events occur before the brightest cluster galaxy is fully assembled, and later mergers alter orbits in six cases, producing diverse evolutionary paths.

What carries the argument

Strongly prolate stellar component used to identify bar-like galaxies, tracked through orbital history relative to brightest cluster galaxy progenitors in the TNG50 simulation.

Load-bearing premise

Galaxies selected by a strongly prolate stellar component are true bars and the simulation correctly models how bars form and respond to tidal forces.

What would settle it

A higher-resolution simulation or observational sample of cluster galaxies showing no systematic bar formation or enhancement tied to pericenter passages around central galaxy progenitors.

Figures

Figures reproduced from arXiv: 2604.01758 by Ewa L. Lokas.

Figure 1
Figure 1. Figure 1: Surface density maps of the stellar component of 15 bar-like galaxies in face-on view at z = 0. Galaxies are identified by their subhalo ID numbers at z = 0, shown on the right of each panel. The color scale is adjusted to the maximum and minimum density of each galaxy and the contours are equally spaced in log surface density. Article number, page 4 [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Evolution of different measures of shape over time for 15 bar-like galaxies. Lines show the axis ratios b/a (blue), c/a (red), the triaxiality parameter T (green), the rotation parameter f (magenta), and the bar mode A2 (black). Galaxies are identified by their subhalo ID numbers at z = 0, shown on the right of each panel. Vertical dashed lines indicate the time of bar formation. Finally, Figs. 5-7 illustr… view at source ↗
Figure 3
Figure 3. Figure 3: Evolution of the total mass over time for 15 bar-like galaxies. Lines show the total galaxy mass in dark matter (blue), stars (red), and gas (green). The dark masses are given in units of 1011 M⊙, while the stellar and gas masses are in units of 1010 M⊙. Galaxies are identified by their subhalo ID numbers at z = 0, shown on the right of each panel. Vertical dashed lines indicate the time of bar formation. … view at source ↗
Figure 4
Figure 4. Figure 4: Distances from perturbers as a function of time for 15 bar-like galaxies. Different colored lines show the distances from distinct perturbers, with data for ID0 shown in blue in all panels. Galaxies are identified by their subhalo ID numbers at z = 0, shown on the right of each panel. Vertical dashed lines indicate the time of bar formation. ID13 The galaxy was a late and short-time visitor to the cluster … view at source ↗
Figure 5
Figure 5. Figure 5: Evolution of the profile of the bar mode, A2(R), over time for five galaxies from the first columns of Figs. 1-4. The galaxies are identified by their subhalo ID numbers at z = 0 given on the right of each panel. The reddest color corresponds to regions with no stars bound to the galaxy. 0 bar mode strength A 2 1 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 time [Gyr] R [kpc] ID14 2 4 6 8 10 12 14 0 2 4 6 8 10 12 1… view at source ↗
Figure 7
Figure 7. Figure 7: Same as [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
read the original abstract

Clusters can provide propitious environments for bar formation in galaxies. This work studies the formation and evolution of 15 bar-like galaxies in the most massive cluster of the TNG50 simulation from the IllustrisTNG suite. The selection includes galaxies from the last simulation output from well-resolved subhalos with a strongly prolate stellar component. Eleven galaxies form or strongly enhance their bars during a pericenter passage around one or more progenitors of the brightest cluster galaxy (BCG). Two form their bars early as a result of minor mergers, one via an interaction with another massive galaxy, and one via disk instability. The bar formation times differ considerably, ranging between 3-11 Gyr. The lengths of the bars also differ, ranging between 2-6 kpc, and do not correlate with the amount of tidal forcing experienced. All galaxies have at least one pericenter passage around a BCG progenitor, but the number of interactions varies strongly and is reflected in the different amount of mass stripping the galaxies experience. Most bar formation events take place before the BCG is fully formed. In three cases, they occur just before different progenitors of the BCG merge. For six bar-like galaxies, the merger events leading to the final formation of the BCG cause significant changes of their orbits. Their diverse evolutionary histories illustrate the different paths to bar formation in clusters and emphasize the complex nature of the process, which includes coevolution with BCG progenitors.

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

1 major / 2 minor

Summary. The manuscript analyzes the formation and evolution of 15 bar-like galaxies in the most massive cluster of the TNG50 simulation. Galaxies are selected at z=0 from well-resolved subhalos exhibiting a strongly prolate stellar component. The central claim is that 11 of these galaxies form or strongly enhance their bars during pericenter passages around one or more progenitors of the brightest cluster galaxy (BCG), while the remaining four form via minor mergers, interactions with other massive galaxies, or disk instability. Bar formation times range from 3-11 Gyr and lengths from 2-6 kpc with no correlation to tidal forcing; all galaxies experience at least one pericenter passage, with varying degrees of mass stripping and orbital changes linked to BCG progenitor mergers. The work emphasizes diverse pathways and coevolution with the BCG.

Significance. If the bar identifications hold, the result provides concrete examples from a high-resolution cosmological simulation of multiple bar-formation channels in dense cluster environments, including the role of tidal encounters with BCG progenitors before the BCG is fully assembled. The direct inspection of simulation snapshots yields falsifiable timing correlations and highlights the complex interplay of interactions, which could inform observational searches for bars in clusters. The use of established TNG50 outputs without additional free parameters is a strength.

major comments (1)
  1. [Abstract and galaxy selection procedure] Abstract and galaxy selection procedure: The identification of the 15 bar-like galaxies rests exclusively on the presence of a 'strongly prolate stellar component' at z=0. In the cluster environment, repeated tidal encounters with BCG progenitors can produce elongated, prolate stellar distributions through stripping and harassment without requiring a rotating bar. No standard bar diagnostics (A2 Fourier amplitude, pattern speed, or buckling signatures) are described to confirm that the prolate shape corresponds to a dynamical bar rather than a tidally induced feature. This directly affects the load-bearing claim that 11 galaxies form or enhance bars specifically during pericenter passages, as the reported timing could instead reflect when tidal distortions become dominant in the inertia tensor or axis ratios.
minor comments (2)
  1. [Abstract] The abstract reports bar lengths ranging 2-6 kpc with no correlation to tidal forcing, but does not specify the exact measurement method (e.g., from stellar density contours or Fourier decomposition) or how tidal forcing is quantified; adding this detail would improve reproducibility.
  2. [Results on evolutionary histories] The manuscript notes that most bar formation events occur before the BCG is fully formed and that merger events cause orbital changes in six cases, but a quantitative summary table of pericenter timings, mass-loss fractions, and bar properties per galaxy would help readers assess the diversity of histories.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for highlighting an important methodological concern. We address the comment on galaxy selection and bar identification below, and we will revise the paper to incorporate additional diagnostics.

read point-by-point responses

  1. Referee: [Abstract and galaxy selection procedure] Abstract and galaxy selection procedure: The identification of the 15 bar-like galaxies rests exclusively on the presence of a 'strongly prolate stellar component' at z=0. In the cluster environment, repeated tidal encounters with BCG progenitors can produce elongated, prolate stellar distributions through stripping and harassment without requiring a rotating bar. No standard bar diagnostics (A2 Fourier amplitude, pattern speed, or buckling signatures) are described to confirm that the prolate shape corresponds to a dynamical bar rather than a tidally induced feature. This directly affects the load-bearing claim that 11 galaxies form or enhance bars specifically during pericenter passages, as the reported timing could instead reflect when tidal distortions become dominant in the inertia tensor or axis ratios.

    Authors: We agree that the current selection criterion based solely on a strongly prolate stellar component at z=0 is insufficient to unambiguously identify dynamical bars, especially given the potential for tidal harassment to produce elongated stellar distributions in the cluster environment. To strengthen the analysis, we will compute the m=2 Fourier amplitude A2(r) for the stellar particles in each galaxy across multiple snapshots, derive pattern speeds where the bar is coherent, and examine vertical buckling signatures in edge-on projections. These standard diagnostics will be applied to all 15 systems to distinguish true bars from purely tidal features. We will update the methods section with a detailed description of these procedures, revise the abstract to reflect the confirmed bar properties, and re-examine the reported formation timings and pericenter correlations using the new metrics. This revision will directly address the concern and provide a more robust foundation for the claims regarding diverse formation channels. revision: yes

Circularity Check

0 steps flagged

Direct inspection of simulation snapshots with no fitted parameters or self-referential derivations

full rationale

The paper selects galaxies from TNG50 outputs based on having a strongly prolate stellar component at z=0 and then traces their histories across snapshots to identify bar formation epochs and interactions. No equations, fitted parameters, or predictions are involved; bar formation times (3-11 Gyr), lengths (2-6 kpc), and pericenter correlations are read directly from the simulation data. No self-citations serve as load-bearing uniqueness theorems, and the analysis does not reduce any claim to its own inputs by construction. This is a standard post-processing study of simulation outputs.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The study rests on the validity of the TNG50 simulation outputs and the assumption that prolate stellar components correspond to bars.

axioms (1)
  • domain assumption TNG50 simulation accurately models galaxy dynamics, bar formation, and tidal interactions in clusters under standard Lambda-CDM cosmology.
    All results derive from interpreting this particular simulation run.

pith-pipeline@v0.9.0 · 5561 in / 1122 out tokens · 47513 ms · 2026-05-13T21:16:11.435351+00:00 · methodology

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