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arxiv: 2606.23674 · v1 · pith:MMQSRVBJnew · submitted 2026-06-22 · 🌌 astro-ph.GA

Ram-pressure signatures in the dwarf irregular galaxy SextansB revealed by deep MeerKAT HI observations

Brenda Namumba , Neel Kolhe , Francois Hammer , Yanbin Yang , Claude Carignan , Roger Ianjamasimanana , Gyula I. G. J\'ozsa , Lourdes Verdes-Montenegro
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Haifeng Wang Hao Chen Xin Huang Fortune Ndalama Amidou Sorgho Marie Korsaga Saul P. Phiri
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Pith reviewed 2026-06-26 08:08 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords Sextans Bdwarf irregular galaxyHI 21 cm observationsram pressureintergalactic mediumLocal GroupMeerKAT
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The pith

Deep MeerKAT HI data show the outer gas disc of Sextans B being disturbed by ram pressure from the diffuse intergalactic medium.

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

The paper reports deep MeerKAT 21 cm observations of the dwarf irregular galaxy Sextans B that reach low column densities and reveal an asymmetric rosette-like filamentary HI structure surrounding a more symmetric stellar disc. Kinematic modelling shows differences between the approaching and receding sides of the rotation curve at large radii. These morphological and kinematic features cannot be produced by stellar feedback and instead match hydrodynamical simulations of ram-pressure stripping by a very low-density IGM. The result provides a second Local Group example, after WLM, that even diffuse intergalactic gas can reshape the outer discs of dwarf galaxies.

Core claim

The combination of morphological and kinematic signatures suggests that the outer HI disc of Sextans B is affected by ram-pressure interaction with the diffuse IGM in the outskirts of the Local Group. This is the second strong example in the Local Group, after WLM, showing that a very low-density IGM can significantly influence the gas distribution and kinematics of dwarf galaxies.

What carries the argument

The asymmetric low-column-density HI envelope with rosette-like filaments together with the divergence between approaching and receding rotation curves at large radii, reproduced by tailored hydrodynamical simulations of IGM ram pressure.

If this is right

  • Ram pressure can operate on dwarf galaxy discs at IGM densities far below those in galaxy clusters.
  • The outer HI discs of other Local Group dwarfs may show similar disturbances from the diffuse IGM.
  • Gas kinematics in dwarfs at large radii should be checked for departures from axisymmetric rotation before attributing them solely to internal processes.
  • Environmental effects on gas content must be considered even for galaxies traditionally viewed as isolated.

Where Pith is reading between the lines

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

  • Surveys of additional low-mass galaxies in the Local Group outskirts could test how common such IGM interactions are.
  • The observed filamentary structures might serve as tracers for mapping the density and velocity structure of the surrounding IGM.
  • If confirmed, this interaction could reduce the retained gas reservoir over time and affect future star formation in Sextans B.

Load-bearing premise

Stellar feedback cannot generate the filamentary HI structure, the asymmetric outer HI envelope, or the divergence between the approaching and receding rotation curves.

What would settle it

Detection of a mechanism internal to the galaxy, such as stellar feedback or past mergers, that reproduces both the rosette-like filamentary HI structure extending to low column densities and the specific divergence in the outer rotation curve sides.

Figures

Figures reproduced from arXiv: 2606.23674 by Amidou Sorgho, Brenda Namumba, Claude Carignan, Fortune Ndalama, Francois Hammer, Gyula I. G. J\'ozsa, HaiFeng Wang, Hao Chen, Lourdes Verdes-Montenegro, Marie Korsaga, Neel Kolhe, Roger Ianjamasimanana, Saul P. Phiri, Xin Huang, Yanbin Yang.

Figure 1
Figure 1. Figure 1: MeerKAT H i column density contours from multiple angular-resolution cubes of Sextans B overlaid on DECaLS gri composite images. This optical image is included for visual refer￾ence only and is not a deep optical map (see [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Highest-resolution MeerKAT H i maps of Sextans B. Panel (a): integrated H i intensity (moment 0). Panel (b): optical stellar emission (red contours, in arbitrary units) overlaid on the H i moment 0 map; the contours are at 5, 10, 20, 40, 80, and 100𝜎 above the background noise (𝜎) of the optical image. Panel (c): intensity-weighted velocity field (moment 1). Panel (d): velocity dispersion map (moment 2). T… view at source ↗
Figure 3
Figure 3. Figure 3: Integrated H i spectrum of Sextans B from MeerKAT (orange), compared with KAT-7 (blue; Namumba et al. 2018) and single-dish measurements (black; Springob et al. 2005; Hunter et al. 2012). The arrow indicates the systemic velocity derived in Section 5. The horizontal dashed grey line marks the zero level. The profile is extracted from the mid-resolution data cube. 4.2. Comparison between the H i and stellar… view at source ↗
Figure 4
Figure 4. Figure 4: Highest-resolution MeerKAT H i integrated intensity map of Sextans B highlighting filamentary structures. Open circle symbols mark the locations of identified H i filaments, which appear as narrow, elongated extensions emerging from the outer disc. The filaments are distributed asymmetrically around the galaxy and are present only in the neutral gas, with no clear counterpart in the stellar or ionized comp… view at source ↗
Figure 5
Figure 5. Figure 5: Radial profiles derived from the TiRiFiC modelling of Sextans B, shown separately for the approaching (blue) and receding [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Comparison between the observed H i velocity field and the best-fit kinematic model derived from the TiRiFiC modelling. Left: intensity-weighted velocity field (moment 1) with observed velocities shown as black contours and model velocities as white contours. Right: residual velocity field computed as the difference between the observed velocity field and the best-fit kinematic model. The model reproduces … view at source ↗
Figure 7
Figure 7. Figure 7: Position–velocity diagrams of Sextans B extracted along the kinematic major axis (left) and minor axis (right). Observed [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Averaged rotation curve of Sextans B before and after [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Evolution of the simulated galaxy. In the top row, we present the evolution of the simulated galaxy described in section [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Simulated dwarf galaxy evolving through the IGM after 8.85 Gyr. Left: A column density map of a simulated dwarf galaxy [PITH_FULL_IMAGE:figures/full_fig_p011_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Radial profiles derived from the TiRiFiC modelling of the H [PITH_FULL_IMAGE:figures/full_fig_p012_11.png] view at source ↗
read the original abstract

The impact of extremely low-density environments such as the diffuse intergalactic medium (IGM) on the neutral gas distribution of dwarf galaxies remains poorly explored observationally. We present deep MeerKAT HI 21 cm observations of the Local Group dwarf irregular galaxy Sextans B that achieve a spectral resolution of 1.4 km/s and reach column-density sensitivities down to 3.3 x 10^18 cm^-2, allowing us to trace the extended HI disc and faint outer structures. The low-column-density HI distribution is asymmetric and reveals a rosette-like filamentary structure superposed on the HI disc. Comparison with the stellar distribution shows offsets between the gaseous and stellar components, with the stellar disc remaining relatively symmetric while the HI envelope becomes increasingly disturbed. 3D kinematic modelling with TiRiFiC reproduces the global velocity gradient but reveals differences between the approaching and receding sides of the rotation curve at large radii, indicating departures from axisymmetric rotation. While stellar feedback can produce small-scale cavities and turbulence in dwarf galaxies, it cannot generate the filamentary HI structure, the asymmetric outer HI envelope, or the divergence between the approaching and receding rotation curves. This is consistent with interaction with a diffuse IGM. Hydrodynamical simulations tailored to Sextans B show that IGM ram pressure acting on the outer gas disc can produce asymmetric gas distributions, filamentary structures, and kinematic perturbations. The combination of morphological and kinematic signatures suggests that the outer HI disc of Sextans B is affected by ram-pressure interaction with the diffuse IGM in the outskirts of the Local Group. This is the second strong example in the Local Group, after WLM, showing that a very low-density IGM can significantly influence the gas distribution and kinematics of dwarf galaxies.

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 / 0 minor

Summary. The manuscript presents deep MeerKAT HI 21 cm observations of the Local Group dwarf irregular galaxy Sextans B, reaching 1.4 km/s spectral resolution and 3.3×10^18 cm^-2 column-density sensitivity. It reports an asymmetric low-column-density HI distribution featuring a rosette-like filamentary structure, offsets between gas and stars, and kinematic departures from axisymmetry revealed by TiRiFiC 3D modeling (differences between approaching and receding rotation curves at large radii). The authors rule out stellar feedback as the origin of these large-scale features and interpret the observations as ram-pressure interaction with the diffuse IGM, supported by tailored hydrodynamical simulations. This is presented as the second such case in the Local Group after WLM.

Significance. If the exclusion of stellar feedback is placed on a quantitative footing, the work would provide valuable observational evidence that very low-density IGM can measurably perturb the outer HI discs of dwarfs, extending the WLM result. The depth of the MeerKAT data and the use of tailored simulations are clear strengths. The central claim remains defensible but currently rests on an assertion rather than a direct test against feedback-only models.

major comments (1)
  1. [Abstract] Abstract (and corresponding discussion section): the statement that stellar feedback 'cannot generate the filamentary HI structure, the asymmetric outer HI envelope, or the divergence between the approaching and receding rotation curves' is load-bearing for the central claim yet is supported only by noting that feedback produces small-scale cavities and turbulence. No feedback-only hydrodynamical run of Sextans B is described that demonstrates these specific large-scale morphological and kinematic features are absent at the quoted sensitivity (3.3×10^18 cm^-2) and resolution (1.4 km/s). The presented simulations are tailored exclusively to IGM ram pressure; an explicit comparison is required to substantiate the exclusion of internal processes.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive feedback, which highlights an opportunity to strengthen the presentation of our central claim. We respond to the single major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract (and corresponding discussion section): the statement that stellar feedback 'cannot generate the filamentary HI structure, the asymmetric outer HI envelope, or the divergence between the approaching and receding rotation curves' is load-bearing for the central claim yet is supported only by noting that feedback produces small-scale cavities and turbulence. No feedback-only hydrodynamical run of Sextans B is described that demonstrates these specific large-scale morphological and kinematic features are absent at the quoted sensitivity (3.3×10^18 cm^-2) and resolution (1.4 km/s). The presented simulations are tailored exclusively to IGM ram pressure; an explicit comparison is required to substantiate the exclusion of internal processes.

    Authors: We agree that the exclusion of stellar feedback rests on a general argument rather than a galaxy-specific feedback-only simulation at the quoted sensitivity and resolution. The manuscript's statement draws from the established literature on dwarf-galaxy feedback, which consistently shows that stellar feedback produces localised cavities and turbulence on scales of tens to a few hundred parsecs, not the kiloparsec-scale filamentary asymmetry and global kinematic divergence reported here. Our tailored ram-pressure simulations reproduce these large-scale features, providing positive evidence for the external-interaction interpretation. To address the referee's concern, we will revise the abstract and discussion to replace the absolute phrasing ('cannot generate') with a qualified statement ('is not expected to generate, given the characteristic scales of feedback in dwarfs') and will add citations to feedback-only simulations of similar dwarfs that lack such extended structures. This constitutes a partial revision; we do not add a new feedback-only run of Sextans B, as that would require a separate modelling effort beyond the scope of the present work. revision: partial

Circularity Check

0 steps flagged

No significant circularity; derivation rests on independent observations and modeling

full rationale

The paper's core argument—that observed HI morphology and kinematics indicate ram-pressure interaction—derives directly from MeerKAT data (column-density maps, velocity fields) and separate hydrodynamical simulations run with IGM ram-pressure parameters. The statement ruling out stellar feedback is an assertion based on scale arguments (feedback produces small-scale features) rather than any fitted parameter, self-defined quantity, or self-citation chain that reduces the conclusion to the inputs. No equations, predictions, or uniqueness theorems are shown to be equivalent to the data by construction. The derivation chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Only the abstract is available; no explicit free parameters, axioms, or invented entities are detailed. The kinematic modeling with TiRiFiC implicitly involves standard disc assumptions and fitted parameters whose values are not reported here.

free parameters (1)
  • TiRiFiC kinematic model parameters
    Rotation curve, inclination, and velocity dispersion parameters are fitted to the HI data cube; specific values and fitting procedure not given in abstract.
axioms (1)
  • domain assumption Standard assumptions of axisymmetric disc geometry and circular rotation in 3D kinematic modeling
    Invoked when using TiRiFiC to reproduce the global velocity gradient and identify departures from axisymmetry.

pith-pipeline@v0.9.1-grok · 5924 in / 1504 out tokens · 30458 ms · 2026-06-26T08:08:37.845908+00:00 · methodology

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