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MeerKAT HI observations of Low Surface Brightness/Ultradiffuse Galaxy Candidates Projected around Two Southern Loose Groups

T0 review · 2 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read Using MeerKAT H I observations, this paper shows that the vast majority of low surface brightness galaxy candidates projected near two groups are not group members but background galaxies, with only two of 52 confirmed as genuine members.

desk verdict Solid new MeerKAT data, two confirmed UDG members, but the conclusion that most non-detected candidates are background is a reasonable hypothesis that the abstract overstates. read the letter →

arxiv 2506.14005 v1 pith:SQOBC2C7 submitted 2025-06-16 astro-ph.CO

Chandreyee Sengupta , Tom C. Scott , Hao Chen , Hyein Yoon , Yogesh Chandola , Mengtian Li , Gyula I. G. Józsa , O. Ivy Wong
show 4 more authors
Yin-Zhe Ma Patricio Lagos Ruta Kale Denis Tramonte
This is my paper
classification astro-ph.CO
keywords lowsurfacebrightnessgalaxiesultradiffuseHI21-cmlineMeerKATgalaxygroupsredshiftsneutralhydrogengroupmembership
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 tests whether low surface brightness galaxies (LSBGs) and ultradiffuse galaxies (UDGs) that appear clustered in projection around two galaxy groups are actually group members. Using sensitive MeerKAT H I observations of 52 candidates, the authors detect neutral hydrogen in only five, all blue LSBGs; two of these are genuine group members (both UDGs), while three are foreground or background galaxies. The authors argue that the roughly 90% non-detection rate, despite sensitivity an order of magnitude below the expected H I content of dwarf galaxies, means most candidates—particularly the blue ones—are not group members but background galaxies. The red candidates' apparent clustering remains unexplained.

What carries the argument

The central mechanism is the H I 21-cm line as a membership test. MeerKAT's sensitivity (per-channel rms of roughly 0.26–0.45 mJy/beam) allows detection of H I masses down to about $10^7$–$2.4\times 10^7$ solar masses at the group distances, an order of magnitude below the H I content expected for field dwarf galaxies of similar optical size. A group-member dwarf should therefore be detected unless severely H I deficient; non-detection at this sensitivity is interpreted as evidence that the galaxy is not at the group velocity.

What would settle it

Measure optical redshifts of the 47 non-detected candidates using deep spectroscopy or high-quality photometric redshifts. If a substantial fraction turn out to have velocities within about 2500 km/s of the group velocities, the non-detection-based conclusion that most candidates are background galaxies would be overturned.

Watch

Extended reading notes

Core claim

The central claim is that the projected clustering of Tanoglidis LSBG/UDG candidates around the groups NGC 1200 and NGC 7396 is largely a line-of-sight projection effect rather than a true three-dimensional association. Only 2 of 52 candidates (about 4%) were confirmed as group members via H I redshifts, and both are blue UDGs. With three-$\sigma$ H I mass sensitivities of roughly $1.0\times 10^7$ and $2.4\times 10^7$ solar masses at the two group distances, the authors conclude that the majority of non-detected candidates, particularly the blue ones, lie at higher redshifts (more than 2500 km/s beyond the group velocities). The red candidates—none detected in H I—still appear clustered, leaving an open question about their nature.

Load-bearing premise

The conclusion that non-detection implies non-membership assumes that any genuine group-member dwarf LSBG would contain enough H I to be detected at MeerKAT's sensitivity; if many candidates are H I-poor despite being group members, the majority could still belong to the groups.

Editorial extensions

If this is right

  • If the conclusion holds, fewer than about 10% of optically selected LSBG/UDG candidates projected near these loose groups are genuine members; the rest are background sources.
  • The two confirmed UDGs are blue and H I-rich, suggesting that H I detections can provide reliable spectroscopic confirmation for at least a subset of UDGs in groups.
  • The unexplained clustering of red LSBGs suggests that either large-scale filamentary structures along the line of sight or selection effects in the red LSBG population are at work.
  • H I-based membership studies can be used to correct statistical samples of UDGs in groups, because photometric projected clustering alone overestimates the true association rate.
  • The lower detection rate compared with Coma-cluster UDG studies (10% versus 25%) indicates that environmental or selection differences between clusters and loose groups affect H I detectability.

Reading between the lines

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

  • If this result generalizes to the full Tanoglidis catalogue of roughly 23,790 candidates, the inferred abundance of UDGs in groups and clusters could be substantially overestimated, with many apparent 'UDGs' being ordinary galaxies at higher redshift that mimic low surface brightness.
  • Optical or near-infrared spectroscopy of the red, H I-undetected candidates could directly test whether the red clustering is produced by a background filamentary structure; a targeted redshift survey of the non-detected candidates would resolve the membership question without relying on H I content assumptions.
  • A testable extension would be to stack the H I spectra of the non-detected candidates to place tighter constraints on their average H I content, distinguishing between a population of H I-poor members and a population of truly background galaxies.
  • Applying the same MeerKAT H I membership procedure to a larger sample of Tanoglidis groups would measure the true group-member fraction statistically and reveal whether the two groups studied here are typical.
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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

2 major / 4 minor

Summary. Using MeerKAT H I observations, this letter searches for 21 cm emission toward 52 LSBG/UDG candidates from Tanoglidis et al. (2021) projected near two southern loose groups, NGC 1200 and NGC 7396. H I is detected in five candidates within ±2500 km/s of the group velocities; two of these, both blue UDGs, are confirmed as genuine group members, while the other three are classified as foreground or background. The paper argues that the 90% non-detection rate, combined with the achieved sensitivity and the expected H I content of dwarf galaxies, implies that the majority of non-detected candidates, especially blue ones, are background galaxies, thereby challenging the reality of the apparent projected clustering.

Significance. The new MeerKAT data represent a substantial improvement over the previous HIPASS search, and the five H I detections are convincingly measured: they are confirmed by the SOFIA2 source finder, and the two group-member UDGs provide valuable spectroscopic confirmations at the group distances. If the background interpretation is correct, the result would be an important caution for LSBG and UDG studies that rely on projected clustering. However, the central inference is not yet fully established: it depends on an unverified prior for the H I content of optically selected LSBGs and on a finite velocity search window. The paper is transparent about several limitations, but the abstract states the conclusion more strongly than the evidence currently supports.

major comments (2)
  1. [§5, point 1 (H I deficiency scenario)] The argument that H I non-detection implies non-membership depends on the field-dwarf scaling from Haynes & Giovanelli (1984) that galaxies with optical diameters 5–10 kpc typically contain 1.8×10^8 to 7.4×10^8 M☉ of H I, but the Tanoglidis LSBG candidates are selected by effective radius and central surface brightness, not optical diameter, and the scatter in H I content among low-mass, low-surface-brightness galaxies at fixed size and color is known to be large. Moreover, the quoted 3σ sensitivities (1.0×10^7 and 2.4×10^7 M☉) are median values for the cubes; Section 4 explicitly states that after primary-beam correction the rms is highest at the map edges. Without per-candidate upper limits, the claim that 'for any dwarf galaxy member of our groups, an H I non-detection implies it is severely H I deficient' is not established for candidates near the beam edge. Please provide per-candidate 3σ M_HI upper limits (or a noise map) and assess the detection rate against a field H I mass function with realistic scatter rather than a single diameter-based expectation.
  2. [§5, 'It could be argued' paragraph] The exclusion of foreground dwarfs rests on the HIPASS non-detections reported in Zhou et al. (2022), but the quoted HIPASS mass sensitivity at 10–20 Mpc (5×10^7 to 2×10^8 M☉) is comparable to or higher than the expected H I mass of the smallest dwarf galaxies, so low-mass foreground galaxies cannot be ruled out by those data. In addition, the MeerKAT search is limited to group velocity ±2500 km/s; a non-detection inside this window is not positive evidence that a galaxy lies beyond it. The abstract states that MeerKAT sensitivity 'allows us to conclude that the majority of the non-detected candidates, particularly the blue galaxies, are not group members but lie at higher redshifts,' while Section 6 more cautiously calls this a hypothesis; given the above, the stronger wording is not supported. Optical spectroscopy of the non-detected candidates or a wider-band H I search would be needed to convert the hypothesis into a demonstrated result.
minor comments (4)
  1. [§6] There are several typographical errors in the conclusions: 'Tangolidis' should be 'Tanoglidis', 'MeeerKAT' should be 'MeerKAT', and 'LSGB/UDG' should be 'LSBG/UDG'.
  2. [Figure 1] Two NGC 1200 blue UDG candidates have very similar coordinates (46.25, -12.47) and appear as a single point; a zoomed inset or separate markers would help the reader identify both candidates.
  3. [§4] The statement that 'stacking of the P1 and P2 spectra did not yield any additional detections' lacks procedural detail; please specify how many spectra were stacked, the resulting rms, and the implied mean H I mass limit.
  4. [Table 1] The uncertainties on the H I line widths (e.g., 34±6 km/s) are not defined; please state how W20 and its error were measured.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the MeerKAT non-detection argument is an empirical inference from external H I-content expectations, not a derivation that reduces to its own inputs.

full rationale

The paper's central claim that most non-detected LSBG candidates, especially blue ones, lie at higher redshifts rests on the absence of H I detections within a finite velocity window combined with external assumptions about field dwarf H I content. This is an empirical inference, not a definitional equivalence: group membership is not defined by H I detection, and the H I mass sensitivity limits are computed from standard radio astronomy formulas and the stated integration times, not fitted to the targets. The only self-citations (Zhou et al. 2022; Sengupta & Balasubramanyam 2006) provide prior HIPASS observations and loose-group dwarf population context; these are independent observational constraints rather than assumptions that already contain the conclusion. The paper explicitly labels the background interpretation a hypothesis, acknowledges the H I-deficiency alternative, and notes that filamentary structures beyond the searched velocity range cannot be ruled out. The skeptical concerns about the H I-content prior, primary-beam rms variation, and finite search window are empirical robustness limitations, not circular reductions. No fitted parameter is renamed as a prediction, and no load-bearing step is justified solely by a self-citation. Thus the circularity score is low.

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

The central claim rests on external H I content expectations for dwarfs (Haynes & Giovanelli 1984) and on the reliability of the Tanoglidis DES catalog and literature group velocities. No free parameters are fitted; the mass sensitivity calculations use standard radio astronomy and stated instrumental parameters.

assumptions (4)
  • domain assumption Field dwarf galaxies with optical diameters 5-10 kpc contain 1.8e8 to 7.4e8 solar masses of H I (Haynes & Giovanelli 1984).
    Used to argue that non-detected candidates would have been detected if they were group members; invoked in Section 5.
  • domain assumption Tanoglidis et al. (2021) DES photometric LSBG/UDG candidates and their colors are reliable.
    Sample definition depends on DES catalog; assumed in Sections 2-4.
  • domain assumption Group systemic velocities (3937 and 4809 km/s) and membership from literature catalogues are correct.
    Used to define +/-2500 km/s search windows and membership classification.
  • standard math UDGs are defined by effective radius >= 1.5 kpc and central surface brightness >= 24 mag/arcsec^2 (van Dokkum et al. 2015).
    Classification criteria applied to the two confirmed H I detections in Section 4.

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Cite this review

Pith. "Pith review of MeerKAT HI observations of Low Surface Brightness/Ultradiffuse Galaxy Candidates Projected around Two Southern Loose Groups." pith.science (2026). https://pith.science/paper/SQOBC2C7

@misc{pith2026250614005,
  author       = {Pith},
  title        = {Pith review of: MeerKAT HI observations of Low Surface Brightness/Ultradiffuse Galaxy Candidates Projected around Two Southern Loose Groups},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SQOBC2C7}},
  note         = {Machine review of arXiv:2506.14005}
}
abstract

A large catalogue of low surface brightness galaxies (LSBGs) from the Dark Energy Survey showed significant clustering around nearby galaxy groups and clusters. Using the HIPASS survey, we tried to determine the redshift of a sub-sample of these LSBGs and determine whether they were members of the groups they were projected near, but this was hampered by HIPASS's high spectral rms. This letter reports on MeerKAT H I observations to determine the redshifts of 52 LSBG candidates projected in the vicinity of two groups from our previous HIPASS study. The main goal is to investigate and ascertain whether these LSBGs are genuine group members. H I was detected with MeerKAT and redshifts were determined for only five of the 52 candidates within a velocity range of $\pm$ 2500 km/s of their respective group velocities. All five H I detections were blue LSBGs and two of them were confirmed to be ultradiffuse galaxies (UDGs). Both these UDGs were group members, while the other three detections were either foreground or background galaxies. In this letter we explore scenarios that can explain the 90% non-detection. MeerKAT's excellent sensitivity allows us to conclude that the majority of the non-detected candidates, particularly the blue galaxies, are not group members but lie at higher redshifts. However, this still leaves the open question as why Tanoglidis LSBG candidates, in particular the red ones, appear to be clustered in projection around nearby groups.

Figures

Figures reproduced from arXiv: 2506.14005 by the authors.

Figure 1
Figure 1. 1 ◦ FWHP primary beams for the MeerKAT pointings are shown with the large green circles and marked P1 and P2 for NGC 1200 (left) and NGC 7396 (right). The projected positions of the LSBG and UDG candidates and their DES g–z colors are indicated by the symbols in the legend. Candidates with an H I detection are shown with mid sized circles and the candidate’s ID number. The circle’s colour indicates the H I detection… view at source ↗
Figure 2
Figure 2. Integrated H I contours overlaid on DES g–band images of the five H I detected UDG candidates, with the galaxy group indicated at the top of each panel followed by its candidate number. The column densities of the contours are as follows (from left to right): image(1) 1.7,2.3,16.0×1019 cm−2 , image(2) 1.8, 4.9, 7.7, 14.3 ×1019 cm−2 , image(3) 2.2 ×1019 cm−2 , image(4) 2.8, 3.7 ×1019 cm−2 , image(5) 1.8 ×1019 cm−2 . … view at source ↗

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Works this paper leans on

17 extracted references · 4 canonical work pages

  1. [1]

    Abbott, T. M. C., Abdalla, F. B., Allam, S., et al. 2018, ApJS, 239, 18, doi: 10.3847/1538-4365/aae9f0 Blue Bird, J., Davis, J., Luber, N., et al. 2020, MNRAS, 492, 153, doi: 10.1093/mnras/stz3357 CASA Team, Bean, B., Bhatnagar, S., et al. 2022, PASP, 134, 114501, doi: 10.1088/1538-3873/ac9642

    doi:10.3847/1538-4365/aae9f0 2018

  2. [2]

    2018, CARTA: The Cube Analysis and Rendering Tool for Astronomy, Zenodo, doi: 10.5281/zenodo.3377984

    Comrie, A., Wang, K.-S., Hwang, Y .-H., et al. 2018, CARTA: The Cube Analysis and Rendering Tool for Astronomy, Zenodo, doi: 10.5281/zenodo.3377984

    doi:10.5281/zenodo.3377984 2018

  3. [3]

    C., Huchra, J

    Crook, A. C., Huchra, J. P., Martimbeau, N., et al. 2007, ApJ, 655, 790, doi: 10.1086/510201 DESI Collaboration, Adame, A. G., Aguilar, J., et al. 2023, arXiv e-prints, arXiv:2306.06308, doi: 10.48550/arXiv.2306.06308 Díaz-Giménez, E., & Zandivarez, A. 2015, A&A, 578, A61, doi: 10.1051/0004-6361/201425267

    arXiv doi:10.48550/arxiv.2306.06308 2007

  4. [4]

    Q., Spekkens, K., Staveley-Smith, L., et al

    For, B. Q., Spekkens, K., Staveley-Smith, L., et al. 2023, MNRAS, 526, 3130, doi: 10.1093/mnras/stad2921

    doi:10.1093/mnras/stad2921 2023

  5. [5]

    P., & Giovanelli, R

    Haynes, M. P., & Giovanelli, R. 1984, AJ, 89, 758, doi: 10.1086/113573

    doi:10.1086/113573 1984

  6. [6]

    E., et al

    Hoosain, M., Blyth, S.-L., Skelton, R. E., et al. 2024, MNRAS, 528, 4139, doi: 10.1093/mnras/stae174 Józsa, G. I. G., White, S. V ., Thorat, K., et al. 2020, in Astronomical Society of the Pacific Conference Series, V ol. 527, Astronomical Data Analysis Software and Systems XXIX, ed. R. Pizzo, E. R. Deul, J. D. Mol, J. de Plaa, & H. Verkouter, 635, doi: 10...

    arXiv doi:10.48550/arxiv.2006.02955 2024

  7. [7]

    2020, ApJ, 902, 39, doi: 10.3847/1538-4357/abb464

    Karunakaran, A., Spekkens, K., Zaritsky, D., et al. 2020, ApJ, 902, 39, doi: 10.3847/1538-4357/abb464

    work page doi:10.3847/1538-4357/abb464 2020

  8. [8]

    D., Condon, J

    Mauch, T., Cotton, W. D., Condon, J. J., et al. 2020, ApJ, 888, 61, doi: 10.3847/1538-4357/ab5d2d 7

    doi:10.3847/1538-4357/ab5d2d 2020

Show all 17 references
  1. [9]

    J., Zwaan, M

    Meyer, M. J., Zwaan, M. A., Webster, R. L., et al. 2004, MNRAS, 350, 1195, doi: 10.1111/j.1365-2966.2004.07710.x

    2004

  2. [10]

    Palumbo, G. G. C., Saracco, P., Mendes de Oliveira, C., et al. 1993, ApJ, 405, 413, doi: 10.1086/172373

    1993 doi

  3. [11]

    J., Barnes, D

    Pisano, D. J., Barnes, D. G., Staveley-Smith, L., et al. 2011, ApJS, 197, 28, doi: 10.1088/0067-0049/197/2/28

    2011 doi

  4. [12]

    M., & Andernach, H

    Plionis, M., Tovmassian, H. M., & Andernach, H. 2009, MNRAS, 395, 2, doi: 10.1111/j.1365-2966.2009.14507.x

    2009

  5. [13]

    C., Brinks, E., Cortese, L., Boselli, A., & Bravo-Alfaro, H

    Scott, T. C., Brinks, E., Cortese, L., Boselli, A., & Bravo-Alfaro, H. 2018, MNRAS, 475, 4648, doi: 10.1093/mnras/sty063

    2018 doi

  6. [14]

    2006, MNRAS, 369, 360, doi: 10.1111/j.1365-2966.2006.10307.x

    Sengupta, C., & Balasubramanyam, R. 2006, MNRAS, 369, 360, doi: 10.1111/j.1365-2966.2006.10307.x

    2006

  7. [15]

    2021, ApJS, 252, 18, doi: 10.3847/1538-4365/abca89 van der Burg, R

    Tanoglidis, D., Drlica-Wagner, A., Wei, K., et al. 2021, ApJS, 252, 18, doi: 10.3847/1538-4365/abca89 van der Burg, R. F. J., Hoekstra, H., Muzzin, A., et al. 2017, A&A, 607, A79, doi: 10.1051/0004-6361/201731335 van Dokkum, P. G., Abraham, R., Merritt, A., et al. 2015, ApJL, ...

    2021 doi

  8. [16]

    E., Pacaud, F., Reiprich, T

    Xu, W., Ramos-Ceja, M. E., Pacaud, F., Reiprich, T. H., & Erben, T. 2018, A&A, 619, A162, doi: 10.1051/0004-6361/201833062

    2018 doi

  9. [17]

    2022, MNRAS, 516, 1781, doi: 10.1093/mnras/stac2344

    Zhou, Y .-F., Sengupta, C., Chandola, Y ., et al. 2022, MNRAS, 516, 1781, doi: 10.1093/mnras/stac2344

    2022 doi

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