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arxiv: 2606.18104 · v1 · pith:O4E4G3GYnew · submitted 2026-06-16 · 🌌 astro-ph.GA

The ultra-deep HI radial profiles of late-type galaxies from MHONGOOSE

S. Veronese , W. J. G. de Blok , E. Brinks , D. Kleiner , S. Kurapati , J. Healy , F. M. Maccagni This is my paper

Pith reviewed 2026-06-27 00:04 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords HI radial profilesneutral hydrogen discsself-similar profileslate-type galaxiesgalaxy environmentMeerKAT observationslow column density gasMHONGOOSE survey
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The pith

HI radial profiles of late-type galaxies become self-similar after scaling to the radius where surface density hits 0.01 solar masses per square parsec.

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

The paper examines the outer edges of neutral hydrogen discs in 16 star-forming galaxies observed with MeerKAT, reaching column densities a few times 10^17 cm^{-2}. It finds that profiles of galaxies spanning three orders of magnitude in HI mass look alike once each is scaled to the radius at which the surface density equals 0.01 solar masses per square parsec. High-mass systems show a flat inner region followed by a knee, while lower-mass ones decline more steeply, yet neither group displays a sharp outer edge. A possible link appears between the scaling radius and local environment, suggesting external processes help set how far the HI extends.

Core claim

The HI radial profiles are self-similar when normalised to the radius at which a mass surface density of 0.01 M_⊙ pc^{-2} is reached. High-mass galaxies exhibit an inner plateau followed by a knee near 5 times 10^{20} cm^{-2}, while low-mass galaxies show steeper declines; no unambiguous edges appear in either case. The normalisation radius itself correlates with environment.

What carries the argument

Normalisation of each galaxy's HI profile to the radius where the azimuthally averaged surface density reaches 0.01 M_⊙ pc^{-2}, after spectral stacking reaches low column densities.

If this is right

  • Diverse HI morphologies arise from a combination of internal and external processes that act differently across galaxy masses.
  • No sharp truncation occurs at the classically debated column density of 10^{19} cm^{-2}.
  • Environment may set the characteristic size of the HI disc in addition to internal galaxy properties.

Where Pith is reading between the lines

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

  • If the self-similarity holds, it implies that the outer HI disc is shaped by a process whose efficiency scales with the total HI reservoir rather than with local star-formation rate.
  • The suggested environmental correlation could be tested by comparing the same normalisation in isolated versus group galaxies matched in HI mass.

Load-bearing premise

Spectral stacking and inclination corrections recover the true low-column-density HI distribution without introducing biases that could create apparent self-similarity or mask real edges.

What would settle it

A larger sample in which the stacked profiles, after the same 0.01 M_⊙ pc^{-2} normalisation, fail to overlap at column densities below 10^{19} cm^{-2} would falsify the claimed self-similarity.

Figures

Figures reproduced from arXiv: 2606.18104 by D. Kleiner, E. Brinks, F. M. Maccagni, J. Healy, S. Kurapati, S. Veronese, W. J. G. de Blok.

Figure 2
Figure 2. Figure 2: Elliptical regions used to derive the H i radial profile for J0309-41. In all panels, the moment 0 map of the galaxy is over￾laid with the 30◦ -wide conical sectors, indicated with magenta lines, used to filter out the disturbances in the disc. In the central panel we overlay also the initial rings (in blue), whereas in the right-side panel we show only the parts of the annuli used to de￾rive the clean H i… view at source ↗
Figure 3
Figure 3. Figure 3: H i radial profile (top row) and its slope (bottom row) for each azimuthal sector for galaxies J1106-14 (left) and J0309-41 (right). The H i radial profile derived from the entire galaxy via stacking is reported with gray points. The points colour-coded with the ring filling factor refer to the H i radial profile derived for the moment 0 map. The coloured lines are the radial profile of each azimuthal sect… view at source ↗
Figure 5
Figure 5. Figure 5: Clean H i radial profiles (top panel) and their slope (bot￾tom panel) on the same radial scale for the 16 MHONGOOSE galaxies studied in this work. The coloured shaded areas are the uncertainty derived from Monte Carlo simulations. panel of [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 2
Figure 2. Figure 2: To take care of the effect of the filling factor of the rings, discussed at the end of Sect. A.2, we considered the H i radial profile derived from the moment 0 map only for rings with fill￾ing factor > 50%. In [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 6
Figure 6. Figure 6: H i radial profile of our galaxies and their column density at which the hydrogen should become mostly ionised. This crit￾ical value is reported, with the same colour of the galaxy, with the horizontal dashed lines. The colour-coding of the galaxies is the same as [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: H i radial profiles (top panels) and their slopes (bottom panels) when normalised to the radius RHI at which the mass surface density reaches 1 M⊙ pc−2 (left panels) and to the radius R001 at which the mass surface density reaches 0.01 M⊙ pc−2 (right panels). The dashed curves refer to the median profile. 4.2. What drives the shape of radial profiles? The lack of evident edges in the H i radial profiles an… view at source ↗
Figure 8
Figure 8. Figure 8: Slope decline (top panel) and normalisation radii (bottom [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
read the original abstract

Galaxy discs have a finite extent, yet how and where their neutral atomic hydrogen (HI) components end is not fully understood. The existence of a break in the HI disc at column densities of $\sim10^{19}$ cm$^{-2}$ has been debated since early 21-cm observations of the spiral galaxy NGC 3198. We present the HI radial profiles of 16 star-forming, late-type galaxies from the MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters (MHONGOOSE) survey spanning a range of HI mass from $10^{7}$ M$_\odot$ to $10^{10}$ M$_\odot$. We probed via spectral stacking their HI discs down to inclination-corrected column densities of a few times $10^{17}$ cm$^{-2}$ at kpc resolution. The HI radial profiles of high-mass (M$_{\rm {HI}}>10^{9}$ M$_\odot$) galaxies are characterised by a inner plateau, followed by a knee at column densities of $\sim5\times10^{20}$ cm$^{-2}$, but no edges are unambiguously identified. The HI radial profiles of low-mass (M$_{\rm {HI}}>10^{9}$ M$_\odot$) galaxies shows a steeper decline and also no edges. We found that the profiles are self-similar when normalised to the radius at which a mass surface density of 0.01 M$_\odot$ pc$^{-2}$ is reached, in agreement with recent literature results, but we also found a possible correlation between the normalisation radius and the environment, suggesting that environment contributes to the shaping of the HI distribution in galaxies. The emerging picture is that the diverse morphology of the HI radial profiles is difficult to interpret, and future studies with a larger sample are necessary for quantifying the contribution of internal and external processes acting at different levels for different 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 / 1 minor

Summary. The manuscript presents HI radial profiles for 16 late-type galaxies from the MHONGOOSE survey, derived via spectral stacking to reach inclination-corrected column densities of a few ×10^{17} cm^{-2} at kpc resolution. High-mass galaxies (M_HI > 10^9 M_⊙) exhibit an inner plateau and knee near 5×10^{20} cm^{-2} with no unambiguous edges; low-mass galaxies show steeper declines, also without edges. The profiles are reported to be self-similar when normalized to the radius at which the HI surface density reaches 0.01 M_⊙ pc^{-2}, consistent with recent literature, with an additional possible correlation between this normalization radius and galaxy environment.

Significance. If validated, the direct observational result would reinforce the self-similarity of HI profiles across galaxy masses when normalized at low column density and indicate a possible environmental role in shaping outer discs. The measurements are direct products with the normalization radius defined from the data itself rather than from a model, avoiding circularity.

major comments (1)
  1. [Abstract and Methods (spectral stacking)] Abstract and Methods (spectral stacking procedure): The self-similarity result and reported absence of edges depend on the stacked, inclination-corrected profiles faithfully recovering the true HI distribution at ~10^{17} cm^{-2}. No mock-data tests, residual maps, or comparisons to individual detections are described to quantify biases from velocity misalignment, weighting schemes, or noise thresholding that could systematically shift the normalization radius (defined at the 0.01 M_⊙ pc^{-2} contour) and thereby induce apparent self-similarity or the environment correlation.
minor comments (1)
  1. [Abstract] Abstract: The low-mass galaxy threshold is stated as M_HI > 10^9 M_⊙, identical to the high-mass threshold; this is evidently a typographical error (likely intended as <) that should be corrected for clarity.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful and constructive review of our manuscript. We address the single major comment below and outline the revisions we will make.

read point-by-point responses

  1. Referee: Abstract and Methods (spectral stacking procedure): The self-similarity result and reported absence of edges depend on the stacked, inclination-corrected profiles faithfully recovering the true HI distribution at ~10^{17} cm^{-2}. No mock-data tests, residual maps, or comparisons to individual detections are described to quantify biases from velocity misalignment, weighting schemes, or noise thresholding that could systematically shift the normalization radius (defined at the 0.01 M_⊙ pc^{-2} contour) and thereby induce apparent self-similarity or the environment correlation.

    Authors: We agree that the manuscript does not describe mock-data tests, residual maps, or direct comparisons to individual detections for the spectral stacking procedure. While the stacking approach follows standard techniques applied in prior HI studies, we acknowledge that explicit validation is necessary to demonstrate that biases from velocity misalignment, weighting, or thresholding do not artificially produce the reported self-similarity or environment correlation. In the revised manuscript we will add a new subsection to the Methods section that presents mock-data simulations incorporating realistic velocity fields, noise characteristics, and weighting schemes. These tests will quantify any systematic shifts in the derived profiles at low column densities and in the normalization radius. We will also include comparisons of stacked profiles against individual detections in the brighter inner discs. These additions will directly strengthen the robustness of the self-similarity result and the suggested environmental dependence. revision: yes

Circularity Check

0 steps flagged

No significant circularity in observational derivation chain

full rationale

The paper reports direct measurements of HI radial profiles obtained via spectral stacking on MHONGOOSE data, with the normalization radius defined empirically as the observed radius where the surface density reaches 0.01 M⊙ pc^{-2}. Self-similarity is presented as an empirical finding after this data-driven rescaling, with no model equations, fitted parameters renamed as predictions, or load-bearing self-citations that reduce the central claim to its own inputs by construction. The derivation consists of observational processing steps that remain independent of the reported similarity result.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claims rest on the accuracy of the MHONGOOSE survey calibration, the validity of spectral stacking for extended low-surface-brightness emission, and the choice of the 0.01 M_⊙ pc^{-2} threshold as a physically meaningful normalization scale. No new entities are postulated.

free parameters (1)
  • normalization surface density threshold
    The radius at 0.01 M_⊙ pc^{-2} is used to scale profiles; its specific value is adopted from recent literature and functions as a chosen reference scale.
axioms (2)
  • domain assumption Inclination corrections accurately convert observed column densities to face-on values without residual projection effects at large radii.
    Invoked when stating inclination-corrected column densities of a few times 10^17 cm^{-2}.
  • domain assumption Spectral stacking recovers the azimuthally averaged radial profile without significant flux loss or bias from velocity gradients or noise properties.
    Central to the claim of probing ultra-low column densities.

pith-pipeline@v0.9.1-grok · 5910 in / 1511 out tokens · 31803 ms · 2026-06-27T00:04:26.126211+00:00 · methodology

discussion (0)

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Reference graph

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