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Searching for HI around MHONGOOSE Galaxies via Spectral Stacking

T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read Stacked radio spectra of six galaxies show almost no neutral hydrogen outside their HI disks down to a column density of 10^17 cm^-2.

desk verdict Careful stacking study with a useful public method and an honest limitations section, but the headline null result is conditional on a kinematic prior validated on only two mocks. read the letter →

arxiv 2411.11584 v2 pith:ISP2AP6O submitted 2024-11-18 astro-ph.GA

S. Veronese , W. J. G. de Blok , J. Healy , D. Kleiner , A. Marasco , F. M. Maccagni , P. Kamphuis , E. Brinks
show 14 more authors
B. W. Holwerda N. Zabel L. Chemin E. A. K. Adams S. Kurapati A. Sorgho K. Spekkens F. Combes D. J. Pisano F. Walter P. Amram F. Bigiel O. I. Wong E. Athanassoula
This is my paper · ORCID
classification astro-ph.GA
keywords neutralhydrogenspectralstackingcircum-galacticmediuminter-galacticMHONGOOSEMeerKATcolumndensitylimitsgalaxygasaccretion
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper argues that the faint neutral atomic hydrogen (HI) that cosmological simulations place in the circum-galactic and inter-galactic medium around Milky Way-like galaxies is not seen even in ultra-deep MeerKAT observations of six nearby star-forming galaxies. By co-adding thousands of spectra after masking out the galaxies' disks, the authors reach limiting column densities near $\sim10^{17}$ cm$^{-2}$ yet recover only a handful of marginal signals, far less than the extended HI structures that the TNG50 mock observations produce. If the result holds, the cool gas that simulations funnel onto galaxies to replenish their star-forming reservoirs must be hidden below this column-density threshold, making direct 21-cm detection of the neutral CGM/IGM implausible even with next-generation radio arrays. The authors present this as a method demonstration plus an observational limit, with an explicit caveat that the spectral alignment depends on an assumption about gas kinematics.

What carries the argument

The load-bearing mechanism is the coupling of a spectral stacker (STACKER) with a one-dimensional line finder (FINDER). STACKER blanks the galaxy and known sources using a detection mask, shifts each line of sight to a reference velocity ('shuffle'), regrids to beam-sized pixels so that Gaussian noise decreases as $\sqrt{N}$ with the number of co-added spectra, and then co-adds spectra inside square cells of $5\times5$, $9\times9$, and $14\times14$ beams. FINDER applies a smooth-and-clip search on the stacked spectra and assigns each candidate a reliability based on the local density of positive versus negative noise peaks in the (peak, sum, mean) flux space, rejecting candidates whose stacked noise fails Gaussianity checks. The crucial kinematic choice is aligning every spectrum to the systemic velocity of the galaxy, which the authors test on two TNG50 mock galaxies and explicitly caution cannot be generalised to arbitrary geometries.

What would settle it

Re-analyse the same full-depth cubes after aligning spectra along the major axis with a flat-rotation-curve velocity field (or a best-fit tilted-ring model) instead of the systemic velocity; if reliable $\sim10^{17}$ cm$^{-2}$ detections appear that disappear under systemic-velocity stacking, the central non-detection is an artefact of the alignment assumption. Alternatively, a deep single-dish radial profile of one MHONGOOSE galaxy reaching below $10^{17}$ cm$^{-2}$ that shows an extended neutral envelope would falsify the claim that the neutral CGM/IGM is essentially absent.

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Extended reading notes

Core claim

On its own terms, the paper's central claim is that unconstrained spectral stacking of the MHONGOOSE data yields essentially no reliable HI emission beyond the detected disks: 13 reliable sources in the full-depth cubes, only four of which have a clear visual counterpart in the data, at integrated fluxes well below what the mock observations would produce. When tested on two TNG50 mock galaxies, the same pipeline recovers only about 35% of the mock HI above $3.6\times10^{17}$ cm$^{-2}$ because the imposed signal-to-noise cut rejects faint stacked lines, so the procedure is not sensitive to everything the simulations put there; yet the mock data still return many more detections than the real data. The authors conclude that the amount of neutral hydrogen outside the HI disk is much smaller than simulations imply, and that the stacked column-density limit of about $\sim10^{17}$ cm$^{-2}$ makes direct emission detection of the neutral CGM/IGM challenging even with future radio telescopes.

Load-bearing premise

The stacking assumes that gas outside the HI disk moves at the galaxy's systemic velocity, so all spectra can be aligned without knowing the true kinematics; if the gas instead co-rotates with the disk or has complex motions, a real signal would be blurred out and the non-detection would not prove the gas is missing.

Editorial extensions

If this is right

  • The neutral atomic hydrogen in the circum-galactic and inter-galactic medium of the six galaxies is below $\sim10^{17}$ cm$^{-2}$, so cool accreting gas is not directly visible as 21-cm emission with current stacking techniques.
  • The HI radial profiles of these galaxies must drop sharply near the disk edge rather than extend as a diffuse neutral envelope, consistent with gas becoming ionised at the measured threshold.
  • Future searches for cool gas accretion will need either absorption-line probes along background quasars or much deeper emission observations than current surveys can provide.
  • The stacking-plus-reliability pipeline itself is transferable to the full MHONGOOSE sample and to other nearby-galaxy data sets once full-depth cubes are available.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • I infer that the systemic-velocity alignment likely sets a floor on what stacking can recover: if the circum-galactic gas co-rotates with the disk, a real signal would be smeared out and the $\sim10^{17}$ cm$^{-2}$ limit would be an artefact of the kinematic assumption rather than evidence of absence.
  • A direct test of that assumption would be to redo the stacking on the same cubes with spectra aligned along the major axis using a flat rotation curve; if reliable detections appear that vanish under systemic-velocity stacking, the paper's non-detection is not the final word.
  • The result, if general, pushes searches for cold accreting gas toward ionised-gas tracers and quasar absorption spectroscopy, since neutral-emission stacking appears to have reached a wall near $10^{17}$ cm$^{-2}$ even when thousands of spectra are co-added.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. This paper presents a spectral-stacking pipeline (STACKER and FINDER) for searching for low-column-density HI emission in the CGM/IGM around MHONGOOSE galaxies. The method is calibrated on two TNG50 mock galaxies, testing different stacking cell sizes, weighting schemes, and line-finder parameters, and comparing two kinematic priors for aligning spectra (systemic velocity vs. co-rotation with the disk). The authors conclude that the systemic-velocity alignment is preferable for their mocks and apply the method to 18 MHONGOOSE galaxies with inclination i≤60°, using single-track cubes for all of them and full-depth cubes for six after quality selection. They report very few reliable detections outside the SoFiA-2 masks, a stacked column-density sensitivity of ~10^17 cm^-2, and conclude that the amount of HI outside the disks is much smaller than predicted by simulations and that direct detection of the neutral CGM/IGM component will be challenging in the future.

Significance. Should the null result hold, it would provide a valuable observational constraint on the cold neutral gas content of the CGM/IGM of nearby galaxies, in a regime where simulations currently predict detectable HI. The paper is methodologically careful in several respects: the noise is checked for Gaussianity, the reliability of detections is assessed through positive-negative source statistics, the method is tested on mock observations, the code is publicly available, and each detection is visually inspected by three independent authors. However, the central inference is conditional on the adopted kinematic prior and on the recovery fraction of the stacking procedure, and the comparison to simulations is only qualitative. These caveats do not invalidate the method paper, but they require that the strongest claims in the abstract and conclusions be appropriately qualified.

major comments (3)
  1. [Sect. 4.2 and abstract] The central null result is conditional on the assumption that all gas outside the SoFiA-2 mask moves at the systemic velocity of the galaxy. The validation of this assumption is limited to two TNG50 mock galaxies, and the authors themselves caution that 'one should be careful not to generalise this result' (Sect. 4.2). For real galaxies with inclination up to 60°, plausible co-rotation of CGM gas with the disk would spread line emission over tens of km/s, lowering the stacked SNR below the FINDER threshold and making the stacked spectra appear empty. The abstract's claim that 'the amount of signal detected outside the HI disk is much smaller than implied by simulations' is therefore not established for the general MHONGOOSE population unless this kinematic dependency is explicitly acknowledged.
  2. [Sect. 4.3] The calibration in Sect. 4.3 shows that only ~35% of simulated emission above 3.6×10^17 cm^-2 is recovered, even under the adopted systemic-velocity prior. Consequently, the quoted ~10^17 cm^-2 stacked limit is a noise-equivalent sensitivity of the stacked spectra, not a demonstrated upper bound on the column density of any gas that may be present in compact or kinematically offset clouds. The abstract and conclusions should clearly distinguish between the sensitivity achieved and the physical limit that can be placed on CGM/IGM HI, otherwise the non-detection is easily overinterpreted.
  3. [Sect. 5.5 and Sect. 4] The statement that the observed signal is 'much smaller than implied by simulations' is not supported by a quantitative comparison. The paper uses two TNG50 galaxies to calibrate the method, but it does not compute an expected detection rate or a predicted stacked SNR for the MHONGOOSE sample from the simulations. The conclusion is therefore a qualitative impression based on two mock galaxies. A quantitative comparison, for example by applying the same stacking procedure to a statistical sample of mock galaxies with the same selection criteria, would be needed to support the claim in the abstract.
minor comments (5)
  1. [Abstract and Sect. 5] The abstract states that full-depth observations are available for '6 nearby star forming galaxies', while Sect. 5 states that full-depth cubes are available for ten galaxies, with six used after quality selection (Table A.1). Please make the wording consistent and explain the selection explicitly.
  2. [Sect. 5.2] The Gaussian tail probabilities appear to be off by a factor of 100: for a Gaussian distribution, P(|F| > 4σ) ≈ 0.0063%, not 0.000063%. The derived excess of ~8.8% is unaffected by this error, but the reported percentages (0.000063% and 0.000069%) should be corrected.
  3. [Throughout (Fig. 7, Fig. B.1, Table 1)] The TNG50 galaxy ID is given as 520885 in Table 1 and in most figure captions, but as 520855 in Fig. 7, Fig. B.1, and the accompanying text. Please unify the notation.
  4. [Sect. 3.2, Eq. (2)] Equation (2) presents a one-dimensional kernel density estimate, while the source parameters are three-dimensional (Fmax, Fsum, Fmean). Please clarify that the KDE is actually applied in the three-dimensional space, or provide the correct multivariate expression.
  5. [Fig. 3 caption] The caption does not explain what the grey-scale background and the black contours represent. Please specify that the background is the masked/regridded cube collapsed along the spectral axis and that the black contours enclose the SoFiA-2 mask.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the stacked null result is an observed measurement compared against external TNG50 predictions; no parameter fitted to the MHONGOOSE data is renamed as a prediction.

full rationale

The derivation chain is self-contained. The MHONGOOSE stacked spectra are actual observational data; the null result is not produced by fitting a parameter to those data. FINDER and STACKER parameters were calibrated on two TNG50 mock cubes, which are external simulation products (Ramesh et al. 2023; Marasco et al. in prep.) and not derived from the target MHONGOOSE galaxies. The kinematic prior (systemic velocity) is an explicit assumption tested on the mocks, with the paper itself cautioning that the result cannot be generalized (Sect. 4.2); this is a limitation on the inference, not a circular reduction of the conclusion to its inputs. The quoted column-density limit is a noise-equivalent sensitivity computed from the stacked noise and cell size, not a fitted quantity. The comparison to simulations is an external benchmark, and agreement with previous independent studies (Das et al. 2024; Xu et al. 2022; Liu et al. 2023) provides outside support. No equation or parameter in the paper reduces the headline claim to a prior definition or a fitted constant.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The central null result rests on four domain assumptions: representativeness of the TNG50 mocks, systemic-velocity kinematics, Gaussian noise, and complete masking. No free parameters are fitted to the MHONGOOSE data to produce the non-detection; the listed parameters were tuned on mocks and set a priori.

free parameters (3)
  • Minimum source SNR = 5
    Sources with integrated SNR below 5 are rejected by FINDER; this cut controls the trade-off between detection and false positives and directly shapes the final source counts and the non-detection result.
  • Stacking cell sizes = 5x5, 9x9, 14x14 beams
    Chosen to balance noise reduction (square root of N) against dilution of compact signals; the sensitivity limits quoted in the paper depend on these sizes.
  • Line finder thresholds = flux threshold 2.5 sigma; smoothing kernels [1,5,9] or [1,7,12]; min linewidth 5 to 7 channels; reliability 0.85
    FINDER parameters were optimized on TNG50 mock cubes (Sect. 4.1) and averaged across galaxies; they determine which stacked features count as reliable detections.
assumptions (4)
  • domain assumption TNG50 mock cubes (Ramesh et al. 2023; Marasco et al. in prep.) are representative of real CGM and IGM HI distributions and kinematics for calibrating the stacking method
    The sensitivity, reliability tuning, and recovery-fraction estimates in Sect. 4 are all derived from these two simulated galaxies.
  • domain assumption CGM and IGM gas in the MHONGOOSE sample moves at the systemic velocity of the host galaxy
    Sect. 4.2 adopts this kinematic prior for all real-data stacking; if gas co-rotates, the alignment smears signal.
  • domain assumption The MHONGOOSE noise is Gaussian for the purpose of reliability statistics
    Sect. 5.2 shows a roughly 8.8 percent excess of voxels above 4 sigma, which the authors argue is symmetric and attributable to artefacts; the reliability calculation assumes Gaussian noise.
  • domain assumption The SoFiA-2 masks successfully remove all emission from the target galaxies and known companions
    Sect. 3.1 blanks lines of sight based on these masks; residual stacked signal is then interpreted as CGM and IGM.

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Cite this review

Pith. "Pith review of Searching for HI around MHONGOOSE Galaxies via Spectral Stacking." pith.science (2026). https://pith.science/paper/ISP2AP6O

@misc{pith2026241111584,
  author       = {Pith},
  title        = {Pith review of: Searching for HI around MHONGOOSE Galaxies via Spectral Stacking},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ISP2AP6O}},
  note         = {Machine review of arXiv:2411.11584}
}
abstract

The observed star formation rates of galaxies in the Local Universe suggests that they are replenishing their gas reservoir across cosmic time. Cosmological simulations predict that this accretion of fresh gas can occur in a hot or a cold mode, yet the existence of low column density ($\sim10^{17}$ cm$^{-2}$) neutral atomic hydrogen (HI) tracing the cold mode has not been unambiguously confirmed by observations. We present the application of unconstrained spectral stacking to attempt to detect the emission from this HI in the Circum-Galactic Medium (CGM) and Inter-Galactic Medium (IGM) of 6 nearby star forming galaxies from the MHONGOOSE sample for which full-depth observations are available. Our stacking procedure consists of a standard spectral stacking algorithm coupled with a one-dimensional spectral line finder designed to extract reliable signal close to the noise level. In agreement with previous studies, we found that the amount of signal detected outside the HI disk is much smaller than implied by simulations. Furthermore, the column density limit that we achieve via stacking ($\sim10^{17}$ cm$^{-2}$) suggests that direct detection of the neutral CGM/IGM component might be challenging in the future, even with the next generation of radio telescopes.

Figures

Figures reproduced from arXiv: 2411.11584 by the authors.

Figure 1
Figure 1. Moment 0 map of the two TNG50 galaxies presented in this paper: 520855 on the left and 555013 on the right. Contours are denoting the Log(NHI) = (16, 19, 21) cm−2 column density in both panels. The field of view has a side-length of 350 kpc, sufficient to encompass the virial radius of both galaxies. 0 h02m 01m 00m 23 h59m 58m 0°30' 15' 00' -0°15' 30' RA DEC 0 h02m 01m 00m 23 h59m 58m -0°30' 15' 0°00' 15' 30' RA DEC… view at source ↗
Figure 2
Figure 2. Moment 1 map of the two TNG50 galaxies presented in this paper: 520855 on the left and 555013 on the right. Black solid-contours denote the receding (50, 100, 150) km s−1 velocities with respect to the systemic velocity. Black dashed-contours instead refer to the approaching (-150, -100, -50) km s−1 velocities. The maps are clipped at the 1016 cm−2 column density level, as this is the noise level we expected to achi… view at source ↗
Figure 3
Figure 3. Stacking regions for J1318-21 reported as blue squares. For the cell size displayed in the figure, STACKER will provide 225 stacked spec￾tra, one for each region. The numbering helps for the bookkeeping. The background grey-scale image is the cube resulting from the preliminary manipulation steps (i.e., masking, shuffling and regridding) collapsed along the spectral axis and the black contours enclose the SoFiA-2 ma… view at source ↗
Figures from the paper (8 more)
Figure 4
Figure 4. Figure 4: Reliability plot of the sources detected in J1318-21 full-depth cube (intermediate cell size). Left panel: Fsum σ vs. Fpeak σ projection of the ( Fmax σ , Fsum σ and Fmean σ ) parameter space. Blue, pink and black points are the positive, negative and reliable sources,…
Figure 5
Figure 5. Figure 5: Comparison between differently extended velocity fields. Left panel: SoFiA-2 moment 1 map of the TNG50 galaxy 520855. Black solid￾contours denote receding (50, 100, 150) km s−1 velocities with respect to the systemic velocity. Black dashed-contours instead refer to app…
Figure 6
Figure 6. Figure 6: Error in the spectral alignment for the TNG50 galaxy 520885 under two different CGM/IGM kinematics assumptions. Left panel: the residuals in terms of |mom1 − v|, where mom1 is the noiseless moment 1 map and v the assumed kinematics, when v = vsys . Light-blue corre￾spo…
Figure 7
Figure 7. Figure 7: Stacking detection map. The background grey-scale image is the cube of the TNG50 520855 galaxy collapsed along the spectral axis and blanked from the galaxy emission. Black contours denote the noiseless moment 0 map clipped at the column density value of 3.6 × 1017 cm−…
Figure 8
Figure 8. Figure 8: Stacking SNR maps for the TNG50 galaxy 520885. First column: stacking regions where SNR > 5 (blue squares) overlaid with the grey￾scale noiseless moment 0 map, blanked from the emission already detected by SoFiA-2. Top to bottom is for cell sizes of 14 × 14, 9 × 9 and …
Figure 9
Figure 9. Figure 9: Comparison between the MeerKAT and Gaussian noise distribu￾tion. Left panel: the pink linear-log histogram represent the number of voxels with a given flux value in a MeerKAT cube, known to contain no bright galaxies. The blue linear-log histogram is instead computed f…
Figure 10
Figure 10. Figure 10: Reliable source around the galaxy J1253-12. The stacked spec￾trum and its 9-channel boxcar smoothed version are given in blue and red, respectively. The detected source is highlighted. The horizontal grey dashed lines are the ±σ level for the unsmoothed spectrum, whil…
Figure 11
Figure 11. Figure 11: Peak flux (left panel), total flux (central panel) and mean flux (right panel) of the stacking detections as a function of their width. Detections in single-track stacked spectra are provided in the background, while detections in full-depth stacked spectra are given …

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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. Full citation record

  1. A search for HI absorption in distant star-forming galaxies with ASKAP-FLASH - II. Direct observations and stacking of 21 cm line

    astro-ph.GA 2026-07 conditional novelty 5.5 of 10

    One new associated HI 21 cm absorber is found at z=0.863; ordered stacking of associated non-detections yields only bootstrap-unsupported tentative features that vanish as fainter sources raise optical-depth noise, wh...

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