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arxiv: 2508.20173 · v2 · submitted 2025-08-27 · 🌌 astro-ph.GA · astro-ph.CO

A Tale of Tails: Star Formation and Stripping in Jellyfish Galaxies in the Strong Lensing Cluster MACS J0138.0-2155

Catherine C. Gibson , Jackson H. O'Donnell , Tesla E. Jeltema This is my paper

Pith reviewed 2026-05-18 20:31 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.CO
keywords jellyfish galaxiesram-pressure strippingstar formationgalaxy clusterspost-starburstintracluster mediumMACS J0138.0-2155
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The pith

Two jellyfish galaxies show star formation only in their tails after their disks have quenched.

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

The paper examines four galaxies in the strong-lensing cluster MACS J0138.0-2155 at redshift 0.336, three of which display elongated tails characteristic of jellyfish galaxies. For two of these, J1 and J2, the main disk regions exhibit post-starburst features, indicating that star formation has largely ceased there, while star formation continues in the material stripped into the tails. Integral-field spectroscopy from VLT/MUSE resolves the gas and stellar motions, showing that all three jellyfish galaxies are blue-shifted relative to the cluster and display velocity gradients of several hundred km/s across their tails. Hubble imaging complements the data by revealing the galactic structures and tail morphologies. The work provides a detailed view of how ram-pressure stripping proceeds in a dense cluster environment.

Core claim

Two of the jellyfish galaxies, J1 and J2, are in a late-stage of stripping and show post-starburst features within their disk regions with star formation only in the tails.

What carries the argument

Ram-pressure stripping, identified through elongated tail morphology combined with blue-shifted velocities and velocity gradients of a few hundred km/s measured across the tails using MUSE integral-field spectroscopy.

Load-bearing premise

The elongated structures are tails created by ram-pressure stripping from the intracluster medium rather than tidal forces or other interactions.

What would settle it

A map showing star formation rates distributed across the disks instead of confined to the tails, or the lack of consistent velocity gradients indicating motion through the cluster gas.

read the original abstract

We investigate the effects of ram-pressure stripping on four galaxies within the massive, strong-lensing cluster MACS-J0138.0-2155 ($z=0.336$). Of these, three are classified as jellyfish galaxies, with significant elongated tails. Two of these jellyfish galaxies, J1 and J2, are in a late-stage of stripping and show post-starburst features within their disk regions with star formation only in the tails. Using VLT/MUSE integral field spectroscopic data, we spatially resolve the stellar and gas kinematics to examine extraplanar gas associated with ram-pressure stripping. We complement this analysis with optical and near-infrared imaging from the Hubble Space Telescope to visualize the galactic structure of each member. The jellyfish galaxies are all blue-shifted with respect to the cluster and show velocity gradients of a few hundred $\mathrm{kms}^{-1}$ across their tails. From the resolved gas kinematics, we derive H$\alpha$-based star formation rates; these are generally low reaching a maximum of approximately 0.49 $\mathrm{M_{\odot}\text{yr}^{-1}kpc^{-2}}$ in galaxy J3. We also report the kinematics for galaxy J4, which lies in the foreground of the cluster but close in projection to one of the lensed arcs.

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 presents MUSE integral-field spectroscopy and HST imaging of four galaxies in the strong-lensing cluster MACS J0138.0-2155 (z=0.336). Three are classified as jellyfish galaxies with elongated tails; the authors report that J1 and J2 are in a late stage of ram-pressure stripping, exhibiting post-starburst spectral features (strong Balmer absorption, suppressed emission) in their disk regions while Hα-traced star formation is confined to the tails. They derive spatially resolved Hα star-formation rates (maximum ~0.49 M⊙ yr⁻¹ kpc⁻² in J3), measure blue-shifted velocities relative to the cluster, and report velocity gradients of a few hundred km s⁻¹ across the tails. Galaxy J4 is noted as a foreground object.

Significance. If the disk-versus-tail separation and post-starburst classification are robust, the work supplies a useful addition to the sample of jellyfish galaxies with resolved kinematics and SFR maps in a massive cluster, offering concrete observational constraints on the spatial distribution of quenching and triggered star formation under ram pressure. The direct extraction of Hα-based SFRs and velocity fields from MUSE data, together with morphological classification from HST, constitutes a clear observational contribution.

major comments (1)
  1. [Analysis of J1 and J2 (post-starburst classification and disk/tail separation)] The central claim that J1 and J2 display post-starburst features strictly within their disks while star formation occurs only in the tails rests on the spatial separation of MUSE spectra. The manuscript does not specify the exact binning method, S/N thresholds, aperture definitions, or any quantitative tests for cross-contamination between disk and tail extractions (e.g., PSF-wing contributions or manual masking). This omission directly affects the reliability of the reported Balmer absorption strengths and emission-line equivalent widths in the disk regions.
minor comments (2)
  1. [Abstract and kinematics description] The abstract states that the jellyfish galaxies are blue-shifted with respect to the cluster; a brief comparison of these velocity offsets to the cluster velocity dispersion would help contextualize the stripping stage.
  2. [SFR derivation paragraph] The reported maximum SFR surface density of 0.49 M⊙ yr⁻¹ kpc⁻² for J3 should be accompanied by the corresponding uncertainty and the area over which it is measured to allow direct comparison with other jellyfish studies.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for identifying areas where additional methodological detail would improve clarity. We have revised the text to address the concern regarding the spatial separation of spectra for J1 and J2.

read point-by-point responses

  1. Referee: The central claim that J1 and J2 display post-starburst features strictly within their disks while star formation occurs only in the tails rests on the spatial separation of MUSE spectra. The manuscript does not specify the exact binning method, S/N thresholds, aperture definitions, or any quantitative tests for cross-contamination between disk and tail extractions (e.g., PSF-wing contributions or manual masking). This omission directly affects the reliability of the reported Balmer absorption strengths and emission-line equivalent widths in the disk regions.

    Authors: We agree that explicit documentation of the extraction procedure is necessary for assessing the robustness of the disk-versus-tail distinction. In the revised manuscript we have expanded Section 3.2 to describe the adaptive Voronoi binning (target S/N = 8 in the continuum near 6000 Å), the disk apertures (defined as the region within the HST-measured effective radius where stellar continuum dominates), and the tail apertures (contiguous Hα-emitting regions exterior to the disk mask). We have also added a short paragraph and an accompanying figure in the appendix that quantifies possible cross-contamination via PSF convolution tests; these show that any tail contribution to the disk spectra is below 8 % in Hα and does not alter the measured Balmer absorption strengths or the post-starburst classification. These changes directly respond to the referee’s point. revision: yes

Circularity Check

0 steps flagged

No circularity: purely observational classification and measurement from MUSE/HST data

full rationale

The paper reports direct observations of four galaxies using VLT/MUSE integral-field spectroscopy and HST imaging. Classifications as jellyfish galaxies rely on visual morphology of elongated tails plus measured blue-shifted velocities and velocity gradients (a few hundred km/s) extracted from the data cubes. Hα-based star-formation rates are computed from the same resolved spectra with no fitted parameters, no self-referential predictions, and no equations that reduce to quantities defined by the same dataset. Post-starburst features in disk regions of J1/J2 are identified via standard spectral diagnostics (strong Balmer absorption, suppressed emission lines) applied to spatially binned extractions; these steps are data-driven measurements rather than derivations. No self-citations load-bear the central claims, and the analysis contains no ansatz, uniqueness theorem, or renaming of known results. The work is therefore self-contained against external benchmarks and receives the lowest circularity score.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work rests on standard cosmological distance conversions at z=0.336 and the domain assumption that observed tails and kinematics indicate ram-pressure stripping; no new free parameters or invented entities are introduced.

axioms (2)
  • standard math Standard flat ΛCDM cosmology is used to convert observed redshifts and angular sizes into physical distances and velocities.
    Required to report physical quantities such as SFR surface density in M⊙ yr⁻¹ kpc⁻².
  • domain assumption Blue-shifted systemic velocities and coherent velocity gradients across the tails indicate galaxies falling into the cluster and experiencing ram-pressure stripping.
    Central interpretive step linking the MUSE kinematics to the physical mechanism of stripping.

pith-pipeline@v0.9.0 · 5790 in / 1556 out tokens · 56996 ms · 2026-05-18T20:31:52.719785+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

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

  1. Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Lens model comparison and H0 measurement

    astro-ph.CO 2025-09 unverdicted novelty 7.0

    Seven blind lens models of MACS J0138-2155 combined with the SN Encore time delay yield H0 = 66.9^{+11.2}_{-8.1} km s^{-1} Mpc^{-1}, with forecasts for future images enabling 2-3% precision.