REVIEW 1 major objections 1 minor 2 cited by
FEASTS: Radial Distribution of HI surface densities down to 0.01 M$_{\odot}$ pc$^{-2}$ of 35 Nearby Galaxies
T0 review · 1 major / 1 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read Deep 21-cm maps show galaxy HI disks are self-similar down to 0.01 M_sun/pc2, with a deep edge predictable from total HI mass to 0.02 dex.
desk verdict A genuinely deeper HI survey and a tight new R001-MHI relation, with an outer-disk geometry assumption that needs scrutiny before the 'universal profile' claim is trusted. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the iso-density radius $R_{001}$, the radius at which the HI surface density reaches 0.01 $M_\odot\,\mathrm{pc}^{-2}$, measured by interpolating deprojected elliptical-annulus profiles from total-power 21-cm images. Normalizing radii by $R_{001}$ collapses all non-tidally-disturbed profiles onto one universal curve, and its tight relation with $M_{\rm HI}$ (slope about 0.5) becomes the new size-mass law. The analytical fit to the median profile uses the double-exponential form $y=\log\left(\frac{I e^{-x/r_{s1}}}{1+(I/J-1)e^{-x/r_{s2}}}\right)$, with $r_{s1}=0.114R_{001}$, along with a cored power-law alternative. The deprojection assumes a thin circular disk whose geometry is set by the $10^{20}$ cm$^{-2}$ contour.
What would settle it
Take a few of the 35 galaxies, including two dwarfs, and observe their outer HI with high resolution and full kinematic information to measure the vertical scale height and warp of the 0.01 $M_\odot$ pc$^{-2}$ emission. If that emission is distributed in a vertically thick or warped layer rather than a flat disk, the thin-disk deprojection underpinning $R_{001}$ fails and the claimed 0.02-dex $R_{001}$-$M_{\rm HI}$ relation and universal profile would need revision. A simpler check is to recompute $R_{001}$ from full 3D kinematic models of the same galaxies and see whether the 0.02-dex scatter persists.
Extended reading notes
Core claim
The paper claims that at surface densities 100 times lower than previously probed, the neutral hydrogen disks of galaxies reorganize into a universal shape. Its central discovery is a new characteristic radius $R_{001}$, the radius at which $\Sigma_{\rm HI}=0.01\,M_\odot\,\mathrm{pc}^{-2}$ (about $10^{18.1}$ cm$^{-2}$). When radial profiles are plotted against $r/R_{001}$, the 35 galaxies, excluding the most strongly interacting, collapse onto one median profile with approximately 0.2 dex scatter; the outer part is exponential with scale-length $0.11R_{001}$. The paper further derives an $R_{001}$-$M_{\rm HI}$ relation, $\log R_{001}=0.49\log M_{\rm HI}-3.11$, with 0.02 dex scatter and essentially the same slope as the known $R_1$-$M_{\rm HI}$ relation, but shifted outward by a factor of about two. The ratio $R_{001}/R_1$ anti-correlates with the HI-to-stellar mass ratio (Pearson $R=-0.66$, $p=0.00$) and with specific star formation rate, but not with stellar mass, HI mass, dark-matter mass, or star formation rate; partial-correlation tests indicate the $M_{\rm HI}/M_*$ relation is the robust one. The authors interpret this as evidence that physical processes cooperate to keep the outer HI disk self-similar, and that gas-rich galaxies transport HI inward from $R_{001}$ to $R_1$ more efficiently, so their disks grow faster near $R_1$.
Load-bearing premise
The load-bearing premise is that the low-column-density HI seen down to 0.01 $M_\odot$ pc$^{-2}$ lies in a thin, circular, coplanar disk whose inclination is set by the $10^{20}$ cm$^{-2}$ contour; if the outer HI is instead a thick, warped, or extraplanar layer, most plausibly in dwarf irregulars, the deprojected surface densities, $R_{001}$, and the claimed universal profile would be systematically biased.
Editorial extensions
If this is right
- A galaxy's deep HI edge can be estimated from its total HI mass alone, to 0.02 dex, without needing deep imaging; this gives observers a direct target for where to look for accretion and stripping.
- Any galaxy-formation model must reproduce a universal, $R_{001}$-normalized outer HI profile with about 0.2 dex scatter and outer scale-length $0.11R_{001}$, independent of stellar mass, halo mass, or star formation rate.
- The $R_{001}/R_1$ anti-correlation with HI-to-stellar mass ratio means gas-richer galaxies concentrate their outer HI more steeply, implying inward transport from $R_{001}$ to $R_1$ is faster in those systems, a quantitative test for gas-regulator or bathtub models.
- Most HI mass lies within $R_1$ (less than 20% outside), so the deep HI distribution is primarily a diagnostic of the disk-CGM interface and gas flows rather than a significant hidden mass reservoir.
- Strong tidal interactions flatten the outer HI profile and inflate $R_{001}$ by about 1.4 times at fixed $M_{\rm HI}$, so the universal relation holds only for galaxies without major interactions, and deviations flag interaction state.
Reading between the lines
- A testable extension: push the same analysis toward the $10^{17}$ cm$^{-2}$ regime, the Lyman-limit boundary. If self-similarity continues, the $R_{001}$-$M_{\rm HI}$ slope should stay near 0.5 and the outer exponential should persist; a break would locate the physical edge set by the ionizing background or CGM pressure.
- The tight $R_{001}$-$M_{\rm HI}$ relation implies that the deep HI edge of a galaxy could be predicted from $M_{\rm HI}$ alone, which could serve as a prior for classifying Lyman-limit absorption systems or planning absorption-line observations toward background quasars.
- For strongly interacting galaxies, the roughly 1.4 times excess in $R_{001}$ at fixed $M_{\rm HI}$ and the flattened profiles offer a potential single-parameter diagnostic of tidal disturbance, applicable to larger samples to identify recent mergers from HI morphology alone.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper presents deep HI surface density radial profiles for 35 nearby galaxies observed with FAST in the FEASTS program, reaching a column density sensitivity of about 10^17.7 cm^-2 (0.004 M_sun/pc^2). The authors define R001 as the radius at which Sigma_HI = 0.01 M_sun/pc^2, and show that, compared to normalization by kiloparsecs, R1, or rvir, the profiles align most tightly when the radius is normalized by R001. From the aligned profiles they derive a 'universal' median profile with a scatter of about 0.2 dex and an outer exponential scale length of 0.11 R001. They also derive a new R001-MHI size-mass relation with a scatter of 0.02 dex and slope 0.49, and report that R001/R1 anti-correlates with HI-to-stellar mass ratio and specific star formation rate for non-interacting galaxies. The paper interprets these results as evidence for self-similar HI disk structure down to 0.01 M_sun/pc^2 and uses them to discuss gas inflow and CGM interaction scenarios.
Significance. If the central claims hold, this is an important observational contribution: it is among the first statistical samples of HI radial profiles extending to surface densities two orders of magnitude below the conventional 1 M_sun/pc^2 level. The proposed R001-MHI relation could become a useful distance-independent size indicator for HI surveys, and the apparent universality of the outer HI profile would provide a strong constraint on models of gas accretion and disk formation. The paper is careful in several respects: it uses total-power FAST data that are not affected by short-spacing problems, it calibrates the beam-smoothing bias using high-resolution combined images, and it provides detailed tables of the measured radii and the median profile. These strengths make the paper valuable for future deep HI surveys with MeerKAT, ngVLA, and SKA. However, the physical interpretation rests on two assumptions that need scrutiny: the thin-disk deprojection at radii well beyond the geometry-defining isophote, and the use of R001 as a normalization, which forces all profiles through one common point by construction.
major comments (1)
- [Section 4 (Discussion)] The interpretation in terms of gas inflow rates (Figure 6) is speculative and goes beyond the data. The paper does not directly measure gas flows; it infers them from the correlation between R001/R1 and MHI/M*. The discussion of 'faster inflow from R001 to R1' is a plausible scenario but is not uniquely constrained by the observations. I recommend softening the language in Section 4 (e.g., 'may indicate', rather than 'implies') and explicitly stating that the kinematic evidence is not yet available. This is a presentation issue, but it affects how the results will be used by the community.
minor comments (1)
- [Abstract] The abstract states that the R001-MHI relation has a scatter of 0.02 dex, but the text in Section 3.2 notes that the standard deviation around the relation is 0.02 dex, while the intrinsic scatter is <0.001. Please be consistent in terminology ('scatter' vs. 'intrinsic scatter') throughout the paper.
Circularity Check
No significant circularity; the central scaling relations are empirical, with only a minor definitional artifact in the R001-normalized profile comparison.
-
self definitional
[Section 2 (definition of R001) and Section 3.1, Figure 2-d]
"We derive the characteristic radius at the ΣHI levels of 1 and 0.01M⊙ pc−2, and refer to them as R1 and R001... when normalized byR001 (Figure 2-d), the profiles align in the tightest way throughout the radius."
R001 is by definition the radius at which every galaxy's ΣHI equals 0.01 M⊙ pc−2, so every R001-normalized profile passes exactly through (r/R001=1, log ΣHI=-2). This forces a degree of alignment in the R001-normalized stack that is absent in kpc or R1 normalizations, so the statement that R001 normalization gives the tightest alignment is partly true by construction. However, the claimed 0.2 dex scatter refers to the full radial range, not just the defining point, and the median profile shape, the fitted scale-length rs=0.11R001, and the empirical R001-MHI relation are not forced by the definition of R001.
full rationale
The paper's central claims are empirical and self-contained against the new FAST data. The R001-MHI relation has a measured scatter of 0.02 dex and is compared with the previously known R1-MHI relation from Wang et al. (2016), rather than being derived from it. The R001/R1 correlations with HI-richness and sSFR are new observational findings with quoted Pearson R and p values. The use of W16 and W24 is supportive context and cross-checks, not a load-bearing derivation: W16 provides an independent interferometric median profile for comparison, and W24 supports the flat-disk morphology claim with direct images. The one genuinely self-referential element is the R001 normalization itself: because R001 is defined as the radius of a fixed surface-density isophote, all profiles coincide at that single point by construction, which somewhat inflates the visual tightness of the R001-normalized stack. This does not propagate to the size-mass relation or the R001/R1 trends, which retain independent empirical content. Overall, the derivation chain is not circular in any load-bearing sense.
Assumptions & free parameters
free parameters (8)
- R001-MHI slope =
0.49 +/- 0.02
- R001-MHI intercept =
-3.11 +/- 0.21
- Profile amplitude I (Eq. 1) =
62.95 (+14.78, -11.13)
- Profile parameter J (Eq. 1) =
5.96 (+0.34, -0.32)
- Inner scale length rs1 (Eq. 1) =
0.114 +/- 0.004
- Outer scale length rs2 (Eq. 1) =
0.106 (+0.007, -0.006)
- Cored power-law core radius rc (Eq. 2) =
0.94 (+0.09, -0.07)
- Cored power-law index beta (Eq. 2) =
8.43 (+1.15, -0.87)
assumptions (7)
- domain assumption The HI layer is a thin disk, so deprojecting by multiplying with the annulus axis ratio (cos i) recovers face-on surface density.
- domain assumption The HI contour at 10^20 cm^-2 provides the correct geometry for all radii down to 0.01 M_sun/pc2.
- domain assumption The FEASTS single-dish data cubes recover all HI flux with no significant missing short-spacing, unlike interferometry.
- domain assumption Distances, stellar masses, SFRs, and group virial radii from external catalogs (z0MGS, Kourkchi and Tully 2017) are accurate.
- domain assumption Manual classification of 11 strongly interacting galaxies and watershed deblending correctly separate galaxy flux.
- standard math The HI emission is optically thin, so 21-cm brightness maps directly to HI column density.
- domain assumption Beam-smoothing corrections derived from 12 combined single-dish plus interferometry images apply to the full sample.
Cite this review
Pith. "Pith review of FEASTS: Radial Distribution of HI surface densities down to 0.01 M$_{\odot}$ pc$^{-2}$ of 35 Nearby Galaxies." pith.science (2026). https://pith.science/paper/MVG2FYL6
@misc{pith2026250101289,
author = {Pith},
title = {Pith review of: FEASTS: Radial Distribution of HI surface densities down to 0.01 M$_\odot$ pc$^-2$ of 35 Nearby Galaxies},
year = {2026},
howpublished = {\url{https://pith.science/paper/MVG2FYL6}},
note = {Machine review of arXiv:2501.01289}
}
abstract
We present the HI surface density ($\Sigma_{\rm HI}$) radial distributions based on total-power HI images obtained by FAST in the FEASTS program, for 35 galaxies with inclinations lower than 72 degree. We derive the HI radius $R_{001}$, which is the radius for the 0.01 $\,M_{\odot}\,{\rm pc}^{-2}$ ($\sim10^{18.1}\,{\rm cm}^{-2}$) iso-density level, 100 times deeper than the 1 $\,M_{\odot}\,{\rm pc}^{-2}$ level previously commonly used to measure $R_1$. The profile shapes show a large diversity at a given radius in units of kpc, group virial radius, and $R_1$, but align more tightly with radius normalized by $R_{001}$. The universal HI profile has a scatter of $\sim0.2$ dex, and a scale-length of $\sim0.11R_{001}$ in the outer region. We derive a new $R_{001}$-$M_{\rm HI}$ relation, which has a scatter of 0.02 dex, and similar slope of $\sim$0.5 as the previously known $R_1$-$M_{\rm HI}$ relation. Excluding strongly tidal-interacting galaxies, the ratio $R_{001}/R_1$ (anti-)correlate strongly and significantly with the HI-to-stellar mass ratio and sSFR, but not with the stellar mass, $M_{\rm HI}$, dark matter mass, or SFR. The strongly tidal-interacting galaxies tend to show deviations from these trends, and have the most flattened profiles. These results imply that in absence of major tidal interactions, physical processes must cooperate so that $\Sigma_{\rm HI}$ distributes in a self-similar way in the outer region down to the 0.01$\,M_{\odot}\,{\rm pc}^{-2}$ level. Moreover, they may drive gas flows in such a way, that HI-richer galaxies have HI disks not only extend further, but also transport HI inward more efficiently from $R_{001}$ to $R_1$.
Figures
Figures from the paper (5 more)
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Reviewed August 10, 2026 · model on record in the stance chip above.
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