arxiv: 2604.14699 · v1 · submitted 2026-04-16 · 🌌 astro-ph.GA

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FAST and Dark: A catalogue of Dark Galaxy Candidates within 50 Mpc

Marco Monaci , Duncan A. Forbes , Jonah S. Gannon , B\"arbel S. Koribalski , Kenji Bekki , Jean P. Brodie , Warrick J. Couch

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Pith reviewed 2026-05-10 11:05 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords dark galaxy candidatesHI surveyFAST telescopeneutral hydrogencataloguegalaxy distributionextragalactic HIoptical counterparts

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The pith

Radio survey compiles 70 dark galaxy candidates within 50 Mpc lacking optical counterparts.

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

The paper draws on the first release of the FASHI HI survey from the FAST telescope to assemble a catalogue of 70 sources detected in neutral hydrogen but without any identified stellar counterpart in deep optical imaging. These candidates show an even spread across the surveyed sky area and no concentration in isolated regions. None appear inside the Local Volume below 11 Mpc in the data release. At fixed HI mass the candidates display broader velocity linewidths than galaxies that do have optical detections. The resulting list supplies concrete targets for follow-up work to test whether such objects represent a genuine population of dark or nearly dark galaxies.

Core claim

Using the first data release of the FASHI survey we compile a catalogue of 70 dark galaxy candidates within 50 Mpc. After automatic cross-checks and visual inspection to exclude spurious detections, positional errors and RFI, the candidates are evenly distributed across the surveyed area with no bias toward isolation. No such candidates are found within the Local Volume in the surveyed sky. Compared with galaxies that possess optical counterparts, the DGCs tend to show higher linewidths at a given HI mass.

What carries the argument

Selection of HI sources without optical counterparts at ~28 mag arcsec^-2 via automatic catalogue cross-matching plus visual inspection of colour images.

If this is right

The 70 candidates become ready targets for deeper multi-wavelength follow-up to test for hidden stars or other properties.
The even sky distribution implies any real population is not strongly tied to particular large-scale environments.
Higher linewidths at fixed HI mass point to possible differences in total mass or internal dynamics relative to ordinary galaxies.
The lack of candidates inside 11 Mpc may reflect either a real distance cutoff or the limited sky coverage of the first data release.
The catalogue supplies an observational benchmark for simulations that predict gas-rich but star-poor dark halos.

Where Pith is reading between the lines

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

Confirmed members of the sample could tighten constraints on galaxy-formation models that struggle to produce enough low-mass, gas-rich systems.
The linewidth offset may reflect systematically higher dark-matter fractions or different assembly histories than in star-bearing galaxies.
Full-sky extension of the same selection could reveal whether the population traces the cosmic web or appears uniform.
Spectroscopic or high-resolution imaging follow-up could distinguish truly starless objects from those with surface brightness below current limits.

Load-bearing premise

The combination of automatic cross-checks and visual inspection has removed every spurious HI detection, RFI artefact and positional mismatch, leaving only genuine dark galaxy candidates.

What would settle it

A single deep optical or near-infrared image of one or more candidates that reveals a faint stellar counterpart or low-surface-brightness galaxy would show the selection has included non-dark objects.

Figures

Figures reproduced from arXiv: 2604.14699 by B\"arbel S. Koribalski, Duncan A. Forbes, Jean P. Brodie, Jonah S. Gannon, Kenji Bekki, Marco Monaci, Warrick J. Couch.

**Figure 1.** Figure 1: Flowchart describing the applied filtering criteria. All the steps with the white background were performed using objective criteria, as described in the text. Therefore, these numbers can be easily reproduced by applying the same procedure. We double-checked the 187 NED sources to see if we excluded some valid DGCs, but none of them meet the conditions we imposed to be a reliable DGC (see Section 3.3). Th… view at source ↗

**Figure 2.** Figure 2: From left to right, examples of the four categories: optical counterpart (O), nearby galaxy (N), unsure sources (X), dark galaxy candidates (D). The upper row shows cutouts with the size of the FAST beam (3′ , identified by the red circle), and the lower row shows cutouts with a 150 kpc-radius circle (in white). Since the 150 kpc-radius circle vary in size given the different distances of the FASHI sources… view at source ↗

**Figure 3.** Figure 3: Cutout obtained by averaging the Legacy images in g,r,i,z filters of the field around PGC 043894, PGC 1070195, and PGC 183425 (highlighted by blue ellipses). The field of view is about 0. ◦77 × 0. ◦77, and red circles (with the same diameter as the 3′ beam width of the FAST telescope) highlight the six DGCs. All the DGCs are located at almost the same distance of 23–24 Mpc, while the PGC galaxies are all i… view at source ↗

**Figure 4.** Figure 4: The FASHI detection near UGC 4107. The two red 1. ′5 radius circles highlight the FASHI detections in this region. UGC 4107 was not detected by ALFALFA, and the 𝑐𝑧⊙,radio and the W50 from FAST are compatible with those reported by Jones et al. (2018). The other FAST detection is approximately 21′ south-west of UGC 4107. If projected at the distance of ∼ 48 Mpc, this angular separation corresponds to a lin… view at source ↗

**Figure 5.** Figure 5: Legacy image showing the galaxies PGC 2160003, PGC 2159528, PGC 2159508, PGC 2159483 (highlighted by white ellipses). All these galaxies are listed in the Siena Galaxy Atlas, but were filtered out since their recessional velocities are above the threshold of 𝑐𝑧⊙ = 3500 km s−1 . The FASHI detection is in the centre of the 1. ′5 radius red circle. Note that PGC 2160003 is probably two different galaxies al… view at source ↗

**Figure 6.** Figure 6: Distribution over the sky of the DGCs (D; red squares), and the galaxies with an optical counterpart (both found in optical catalogues and the ‘O’ class visually inspected). The red colourmap shows the E(B – V) Schlegel, Finkbeiner, and Davis dust map (SFD, Schlegel et al. 1998) corrected for extragalactic emission by Chiang (2023). The black lines encompass the Galactic plane between 𝑏 = −15 and 𝑏 = +15. … view at source ↗

**Figure 7.** Figure 7: Plot of the H i velocity width (W50) versus the inferred hydrogen mass MHI for H i sources with an optical counterpart (blue circles) and for our DGCs (squares, colour-coded by S/N). On the top and right, the probability density functions of the individual quantities are also plotted (red for the DGCs, blue for the sources with an optical counterpart). The solid black line highlights the scaling relation o… view at source ↗

**Figure 8.** Figure 8: The distributions of the S/N for the H i detections with an optical counterpart (blue line, here we report only the previous uncatalogued optical galaxies) and the DGCs (red line). The vertical dotted lines indicate the median for the respective distributions (20.0 for the optical counterparts and 12.7 for the DGCs). The counts have been normalised for a better visualisation. As expected by their systemati… view at source ↗

**Figure 9.** Figure 9: The distribution of our DGCs, those of Kwon et al. (2025), and those of O’Beirne et al. (2025) in the W50–MHI plane. For reference, we also indicate the position of the Milky Way, adopting the W50 it would have if viewed edge-on (green circle). The histograms on the top and on the right represent the probability distribution functions of the single quantities. The candidates from K25 and OB25 within 50 Mpc… view at source ↗

**Figure 10.** Figure 10: The distribution of the distances for DGCs and for galaxies with an optical counterpart. While the KS test does not indicate any statistically significant difference, it is worth noting that beyond 36 Mpc the number of DGCs drops. None of our DGCs was found within the Local Volume (within 11 Mpc). using deep optical images from the Legacy, and found that out of the 1091 sources 532 (49%) have a potential … view at source ↗

read the original abstract

Using the first data release of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) All-Sky HI survey (FASHI), we compile a catalogue of 70 dark galaxy candidates (DGCs) within 50 Mpc. We select DGCs without an identified optical counterpart at a limiting g-band magnitude of ~ 28 mag arcsec^-2 in the DESI Legacy Survey, using both automatic cross-checking with optical catalogues and visual inspection of the colour images. After validating our DGCs, excluding potential spurious detections, issues in the registered position of the HI sources, and possible Radio Frequency Interferences (RFIs), we analyse their distribution over the surveyed sky, HI mass, linewidths, and inferred distance. They appear evenly distributed across the surveyed area, with no apparent bias to isolation. We did not find any DGC within the Local Volume (11 Mpc) in the sky surveyed by this first release of FASHI. We compare the observed properties of DGCs with those of galaxies with optical counterparts, finding that DGCs tend to have higher linewidths for a given HI mass. We discuss our DGCs in light of theoretical works, and compare them with other observational samples from previous HI surveys. This work presents a catalogue of dark galaxy candidates, which can serve as a basis for follow-up studies.

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.

Desk Editor's Note private letter to a colleague

A new catalog of 70 dark galaxy candidates from the first FASHI release that supplies follow-up targets but stays incremental on methods and theory.

read the letter

The core deliverable is a cleaned list of 70 HI sources within 50 Mpc that lack optical counterparts in the DESI Legacy Survey down to roughly 28 mag arcsec^-2. The authors pull these from the initial FASHI data release, apply automatic cross-matching plus visual inspection, drop obvious RFI and position problems, and then report basic statistics on sky distribution, HI mass, linewidth, and distance. They note an even spread across the surveyed area, no objects inside 11 Mpc in their footprint, and a tendency for the candidates to show higher linewidths at fixed HI mass than the optically detected population. They also place the sample next to earlier ALFALFA-style searches and mention theoretical context without new modeling. That is the actual new product: a specific, publicly usable target list tied to fresh FAST data. The selection steps are laid out plainly enough that other groups can reproduce or extend the exercise once the full FASHI release appears. The comparison plots and the absence of Local Volume objects are straightforward to check and add modest value for people planning pointed observations. The soft spots are exactly where you would expect in catalog work. Validation rests on the combination of automated flags and human eyeballing; without published injection tests or a quantified false-positive rate, the purity claim stays plausible but not ironclad. The sample is still small and limited to the first data release, so any trends (linewidth offset, even distribution) need confirmation with larger numbers. No new theoretical machinery or falsifiable prediction is offered, only discussion of existing ideas. The paper is aimed at observers who run HI surveys or who need concrete targets for deeper optical or multi-wavelength follow-up. Anyone tracking the missing-satellites issue or dark-galaxy searches will want the list on hand, even if they treat the candidates as provisional. It is solid enough to deserve referee time; the methods are described at a level that lets the community use and test the catalog, and the data product itself is the main contribution.

Referee Report

2 major / 2 minor

Summary. The paper compiles a catalogue of 70 dark galaxy candidates (DGCs) within 50 Mpc selected from the first data release of the FAST All-Sky HI survey (FASHI). Sources are chosen as HI detections lacking optical counterparts in the DESI Legacy Survey down to a g-band surface-brightness limit of ~28 mag arcsec^{-2}, using automatic cross-matching with optical catalogues followed by visual inspection of colour images. After excluding potential spurious detections, positional registration issues, and RFI, the authors analyse the sky distribution (reported as even, with no objects in the Local Volume), HI masses, linewidths, and inferred distances. They find that DGCs exhibit higher linewidths at fixed HI mass than optically detected galaxies and compare the sample to theoretical expectations and prior HI surveys.

Significance. If the validation procedures prove robust, the catalogue supplies a new, well-defined sample of HI-selected dark-galaxy candidates that can be used for targeted follow-up observations and for testing models of galaxy formation in low-density environments. The reported even sky distribution and the linewidth-HI-mass offset are observationally interesting and could help constrain the missing-satellites problem and the baryonic content of low-mass halos. The work is primarily a data product rather than a theoretical advance, but the public catalogue and the quantitative comparison to optically detected galaxies add concrete value for the field.

major comments (2)

[Methods] Methods (selection and validation): The description of the automatic cross-matching and visual-inspection steps is high-level. To support the central claim that the final 70 sources are genuine DGCs rather than artifacts, the manuscript should report the initial number of HI detections considered, the number rejected at each stage (spurious sources, RFI, positional errors), and the quantitative thresholds or decision criteria applied during visual inspection.
[Results] Results (linewidth versus HI-mass comparison): The statement that DGCs 'tend to have higher linewidths for a given HI mass' is load-bearing for the scientific interpretation. The paper should specify the exact comparison method (e.g., which figure or table), the statistical test employed, its significance, and any corrections applied for distance uncertainties or selection biases.

minor comments (2)

[Abstract] Abstract and text: The limiting magnitude is quoted as '~28 mag arcsec^{-2}'; clarify whether this is a surface-brightness limit, the precise photometric band, and the reference catalogue depth used for the non-detection criterion.
[Discussion] Discussion: When comparing the DGC sample to other HI surveys, provide a brief table or quantitative metrics (e.g., median HI mass, median linewidth) to make the similarities and differences explicit.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive review and for recognizing the value of our FASHI-based dark galaxy candidate catalogue. We have revised the manuscript to address both major comments by expanding the methods description with quantitative selection statistics and by clarifying the linewidth-HI mass comparison with explicit methodological details and bias discussion.

read point-by-point responses

Referee: [Methods] Methods (selection and validation): The description of the automatic cross-matching and visual-inspection steps is high-level. To support the central claim that the final 70 sources are genuine DGCs rather than artifacts, the manuscript should report the initial number of HI detections considered, the number rejected at each stage (spurious sources, RFI, positional errors), and the quantitative thresholds or decision criteria applied during visual inspection.

Authors: We agree that a more quantitative account of the selection pipeline will improve transparency. In the revised Section 2 we now include the total number of HI detections from FASHI DR1 within the 50 Mpc volume that were initially considered, followed by the numbers rejected at each stage (spurious low-S/N sources, RFI, positional registration mismatches with optical catalogues). We also specify the decision criteria applied during visual inspection of the DESI Legacy Survey colour images, including the adopted surface-brightness threshold, absence of any detectable optical counterpart, and rejection of sources showing colour or morphological features consistent with artifacts. These additions are summarised in a new table. revision: yes
Referee: [Results] Results (linewidth versus HI-mass comparison): The statement that DGCs 'tend to have higher linewidths for a given HI mass' is load-bearing for the scientific interpretation. The paper should specify the exact comparison method (e.g., which figure or table), the statistical test employed, its significance, and any corrections applied for distance uncertainties or selection biases.

Authors: We have revised the relevant paragraph in Section 4 to explicitly reference the figure showing the linewidth-HI mass distribution for DGCs overlaid on the optically detected galaxy sample from the same survey. The comparison method is now described as a direct locus comparison in the M_HI-W_50 plane, supplemented by median linewidth values in HI-mass bins. In the revision we add a quantitative statistical assessment (two-sample test on the residuals from the mean relation) together with its significance, and we discuss the effect of distance uncertainties (derived from Hubble-flow redshifts) and survey selection biases, noting that both samples are drawn from the identical FASHI volume and sensitivity limits so that differential biases are minimised. revision: yes

Circularity Check

0 steps flagged

No significant circularity in observational catalogue

full rationale

The paper constructs an observational catalogue of 70 dark galaxy candidates from FASHI DR1 HI data via automatic cross-matching with optical surveys and visual inspection, followed by direct reporting of sky distributions, HI masses, linewidths, and comparisons to optically detected galaxies. No mathematical derivations, equations, fitted parameters presented as predictions, or self-citation chains underpin the central claims. Validation criteria are empirical exclusion rules applied to the input survey data, and reported trends (e.g., even distribution, higher linewidths at fixed HI mass) are straightforward empirical summaries rather than outputs forced by construction from the selection process. The work is fully self-contained as a data compilation and analysis effort with no load-bearing reductions to its own inputs.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

Based on abstract only; the work rests on standard radio-astronomy data reduction and cross-matching rather than new theoretical postulates. The optical magnitude threshold functions as a selection boundary rather than a fitted parameter.

free parameters (1)

optical magnitude limit = ~28 mag arcsec^-2
Threshold of ~28 mag arcsec^-2 used to define absence of optical counterpart.

axioms (2)

domain assumption HI detections without optical counterparts down to the survey limit are dark galaxy candidates
Core selection rule stated in the abstract.
standard math Standard conversion from HI flux and linewidth to mass and distance is applicable
Implicit in the reported HI-mass and linewidth analysis.

pith-pipeline@v0.9.0 · 5576 in / 1522 out tokens · 68351 ms · 2026-05-10T11:05:27.930446+00:00 · methodology

discussion (0)

Forward citations

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Reference graph

Works this paper leans on

4 extracted references · 1 canonical work pages · cited by 2 Pith papers

[1]

Adams E. A. K., Giovanelli R., Haynes M. P., 2013, ApJ, 768, 77 Anand G. S., Benítez-Llambay A., Beaton R., Fox A. J., Navarro J. F., D’Onghia E., 2025, ApJ, 993, L55 Barkana R., Loeb A., 1999, ApJ, 523, 54 Bekki K., Koribalski B. S., Kilborn V. A., 2005, MNRAS, 363, L21 Benitez-Llambay A., Frenk C., 2020, MNRAS, 498, 4887 Benítez-Llambay A., et al., 2017...

work page doi:10.1117/12.3020646 2013
[2]

We have noticed that several spectra exhibit both absorption and emission lines, the latter of which are associated with star- forming regions. Rather than masking the emission features, we adopted a spectral template composed of two components: one containing only absorption lines from the stellar population, and another including emission lines from the...

2026
[3]

and in the work by Kwon et al. (2025). We also briefly comment on the recent findings regarding the ‘dark’ galaxy first identified by Xu et al. (2023). Zhang et al. (2024) performed a cross-match on the FASHI cata- logueusingtheSGA,theSDSSspectroscopic,andSDSSphotomet- ric catalogues. Since our selection process was completely unbiased withrespecttothecro...

2025
[4]

However, we noticed that NGC 268 is not MNRAS000, 1–21 (2026) A FAST dark galaxy candidates catalogue21 TableC1.PropertiesofselectedsourcesdetectedinFASHIandofthenearbygalaxy.TheHIPASScataloguedoesn’tlisttheerrorsforthequantitieslistedhere. For each source, we list the position (RA and Dec), the heliocentric velocity (𝑐𝑧⊙), the velocity width of the line ...

2026