REVIEW 2 major objections 6 minor 288 references
Atomic gas dominates six Local Group galaxies, and its depletion time rises to tens–hundreds of Gyr in the outer disks, indicating that star formation is throttled by the slow conversion of atomic to molecular gas.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-01 06:32 UTC pith:HFNM5N7N
load-bearing objection A careful, transparent data-release paper with real new products; the headline depletion-time trends are plausible but partly rest on SFR upper limits, so the strongest conclusions should be softened. the 2 major comments →
A Multiwavelength Inventory for the Local Group L-band Survey I: Atlas and Radial Profiles of Local Group Galaxies
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper's central claim is that in the six actively star-forming Local Group galaxies targeted by LGLBS, atomic hydrogen makes up most of the interstellar medium and, in the outer regions, exceeds the stellar mass. Azimuthally averaged HI surface-density profiles are flat or slowly declining compared to the steeper falloff of stellar mass and star-formation rate, with outer HI scale lengths of 1–8.5 kpc. Atomic and total gas depletion times increase with galactocentric radius, rising from roughly 0.8–17 Gyr inside the stellar half-mass radius to tens–hundreds of Gyr in the outer disks, while molecular depletion times remain shorter and roughly constant where CO data exist. Outside the stel
What carries the argument
The central object is the azimuthally averaged radial profile, computed at 120 pc resolution for gas, stars, and star-formation tracers, along with the mass-weighted mean atomic gas surface density ⟨Σ_120pc_atom⟩ and the associated clumping factor c_HI. The key derived quantity is the gas depletion time τ_dep = Σ_gas / Σ_SFR (the inverse of the star-formation efficiency per unit gas), which the paper tracks as a function of radius. These tools convert raw multiwavelength maps into a direct measurement of where and why star formation becomes inefficient, showing that the atomic-to-molecular gas balance, not the molecular depletion time, sets the pace of star formation.
Load-bearing premise
In all targets except M31, the star-formation tracers (UV, IR, Hα) are not sensitive enough to detect the outer-disk star formation, so many radial rings yield only upper limits; if the true outer-disk SFR is systematically higher than these limits, the reported long depletion times would be overestimated and the central conclusion weakens.
What would settle it
Deeper ultraviolet or Hα observations that push star-formation detection into the outer HI disks of IC 10, IC 1613, NGC 6822, WLM, or M33 — or dust-based molecular gas maps that could reveal CO-dark H2 — would directly test whether τ_atom_dep really exceeds 10 Gyr outside the stellar effective radius.
If this is right
- In all six galaxies, atomic gas dominates the ISM and exceeds stellar mass in the outer regions, so models of disk-galaxy evolution must treat the HI reservoir as the primary fuel reservoir.
- Atomic and total gas depletion times rise exponentially with radius in every target, implying a continuous, non-threshold transition to inefficient star formation in outer disks.
- Outside the stellar effective radius, τ_atom_dep is always > 10 Gyr, meaning the majority of the atomic gas by area is currently forming stars far too slowly to consume itself within a Hubble time.
- The clumping factor shows that dwarf galaxies are highly porous (shells and cavities) while M31 and M33 are smoother, linking the ISM structure on 120 pc scales to the efficiency of forming molecular clouds.
- The public release of calibrated HI, CO, IR, UV, Hα, stellar-mass, and SFR maps provides a foundation for high-resolution studies of the HI-to-H2 transition in the nearest galaxies.
Where Pith is reading between the lines
- If the HI-to-H2 transition is the universal bottleneck, then cosmological simulations that adopt a fixed star-formation efficiency per unit gas will overproduce stars in the outer disks of low-mass galaxies; this prediction is testable by comparing simulated and observed radial SFR profiles.
- The absence of a sharp threshold in the depletion-time rise suggests that local pressure or column density, rather than a single critical value, controls where molecular gas forms; stacking HI spectra with higher resolution could map the transition directly.
- A direct extension would be to measure the CO-dark molecular gas fraction in these outer disks using dust-based mass estimates, which would test whether the long atomic depletion times are truly due to inefficient HI-to-H2 conversion or partly to a missed molecular reservoir.
- The observed correlation between clumping factor and slow star formation implies that feedback-generated holes may self-regulate the HI-to-H2 conversion; this could be tested by comparing cold-HI fraction (from absorption) with local depletion time on 120 pc scales.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper combines 120 pc-resolution VLA+GBT HI data from the first LGLBS data release with uniformly processed GALEX UV, WISE IR, Hα, and CO maps to construct radial profiles of atomic gas, molecular gas, stellar mass, and star formation rate surface density for six Local Group galaxies: M31, M33, IC 10, IC 1613, NGC 6822, and WLM. From these profiles the authors measure disk scale lengths, effective radii, gas clumping factors, integrated masses, SFRs, and atomic/molecular/total gas depletion times. The central claims are that atomic gas dominates the ISM over the whole sample, that atomic and total gas depletion times increase with galactocentric radius, and that outside the stellar effective radius the atomic-gas depletion time always exceeds 10 Gyr. The paper also releases the processed images, profiles, and region measurements as a public data product.
Significance. If the conclusions hold, this is a valuable reference dataset and analysis for the nearest HI-resolved galaxies. The 120 pc resolution of the HI data is a clear step forward for studying the HI-to-H2 transition and star formation inefficiency in HI-dominated systems. The uniform multi-wavelength processing and public data release are significant community contributions. The central HI and stellar mass measurements are independent of the depletion-time assumptions and are robust. However, the headline radial trend of increasing depletion times rests heavily on the treatment of SFR upper limits in outer disks; unless those are handled with censored-data methods or explicitly labeled as lower limits, the quantitative conclusions go beyond what the data support.
major comments (2)
- [Section 2, Table 4, Conclusions 2-3] Section 2 states that 'in all targets except M31, the sensitivity of the SFR tracers becomes a limiting factor in the outer disks... so that many rings yield only upper limits.' Yet the depletion-time panels and Table 4 quote numerical τ values (e.g., IC 1613 τ_gas_dep(r>r50,*)=49.85 Gyr; WLM τ_gas_dep=16.44 Gyr) without marking them as lower limits. Since SFR sensitivity per annulus improves outward while Σ_HI declines only slowly, the ratio Σ_HI/SFR_UL can rise with radius even for a constant true SFR. Therefore the claimed 'increase with radius' and 'always >10 Gyr' trends are not established for the censored rings unless survival analysis/stacking is applied or the values are explicitly labeled as lower limits and fits restricted to detections. This is load-bearing for Conclusions 2 and 3.
- [Appendix A.4, Conclusion 1] For IC 1613, NGC 6822, and WLM, the 'expected' molecular gas profiles (and hence the statement that Σ_atom > Σ_mol in every ring) are computed by assuming a single τ_mol_dep per galaxy, derived by extrapolating the Sun et al. (2025) τ_mol_dep–M* relation from M* ≈ 10^9.5–10^11 M⊙ down to M* ≈ 10^7.5–10^8.8 M⊙. This extrapolation is uncalibrated and the resulting M_mol values in Table 5 are not independent measurements. The conclusion that atomic gas dominates over molecular gas in these dwarfs should be presented as conditional on this assumed τ_mol_dep, and the uncertainty in the extrapolation should be propagated into the reported M_mol and any statements about molecular subdominance.
minor comments (6)
- [Table 5 note] The note 'The mean fractional SFR uncertainty across the sample is 0.077%' is misleadingly precise. It reflects only propagated random noise in the profiles, not the systematic uncertainties from SFR tracer calibration, background subtraction, or the upper-limit status of outer-disk rings. Please quote a systematics-inclusive uncertainty or remove the value.
- [Figure captions (Figs. 2, 4, 6, 8, 10, 12)] The vertical 'FUV threshold' (and 'W4 threshold') lines are not defined in the captions. Please state explicitly that points beyond these thresholds are 3σ upper limits in Σ_SFR and thus lower limits in depletion time.
- [Table 4] The table would benefit from explicit '>' or 'lower limit' markers on entries where the SFR is an upper limit, so that the table is self-consistent with the stated data limitations.
- [Table 2] The header 'Hirot.' appears to be a garbled abbreviation; it should read 'HI rotation curve' for clarity.
- [References] The Koch et al. (2025) reference has a malformed author list ('Koch, E., LGLBS, C., LGLBS, C., ...'). Please fix it.
- [Appendix B] The W1-based contamination slopes α for W4 and FUV in M31 are quoted without uncertainties. Since this correction affects the central M31 SFR profiles, an uncertainty estimate would be helpful.
Circularity Check
HI-dominance conclusion partly rests on 'expected' molecular gas built from SFR and an assumed depletion time; the central τ_dep results use independent HI and SFR data.
specific steps
-
fitted input called prediction
[Appendix A.4 / Eq. A6; Table 5 note b; Section 4 Conclusion 1]
"We estimate the molecular gas surface density expected based on the star formation rate surface density in each ring. ... Σmol(τ mol dep ) M⊙ pc−2 = 10 3 ΣSFR M⊙ yr−1 kpc−2 τ mol dep Gyr (A6) ... When CO measurements are not available or not constraining, we estimate M mol from the Σ SFR profile and τ mol dep. ... Except for the innermost part of the dwarf starburst IC 10, Σ atom > Σ mol in every azimuthally averaged ring in our sample."
For IC 1613, NGC 6822, and WLM the quoted Σ_mol is not measured but constructed via Eq. A6 from the same Σ_SFR that enters τ_atom_dep and an assumed τ_mol_dep (0.65–0.7 Gyr from Sun et al. 2025). Since Σ_mol_expected/Σ_atom = τ_mol_dep/τ_atom_dep, Conclusion 1's claim that Σ_atom > Σ_mol in these galaxies is algebraically equivalent to asserting the assumed molecular depletion time is shorter than the measured atomic depletion time. The paper labels these profiles 'expected,' which mitigates the issue, but the HI-dominance result for these three dwarfs is not an independent molecular-gas constraint.
full rationale
The central depletion-time analysis is not circular: τ_dep = Σ_gas/Σ_SFR is computed from LGLBS HI data and uniformly processed GALEX/WISE/Hα SFR maps, and for galaxies without CO data the paper explicitly takes Σ_gas = Σ_atom (Table 4 note a). No equation in that main derivation reduces to its own inputs. The self-citations (Koch et al. 2025 for HI data, Sun et al. 2022 for profile code, Sun et al. 2025 for the τ_mol_dep scaling relation) are data/calibration references, not load-bearing uniqueness theorems, and the results are compared against independent literature. The important limitation flagged in Section 2 — 'in all targets except M31, the sensitivity of the SFR tracers becomes a limiting factor in the outer disks of our targets so that many rings yield only upper limits' — is a sensitivity/statistical concern, not a circular construction: using upper-limit SFR as a detection can make apparent τ_dep rise with radius through the noise floor and makes Table 4 outer-disk values lower limits, which should be weighed in correctness. The one genuinely constructed element is the 'expected' molecular gas estimate of Eq. A6, used as support for the HI-dominance conclusion; because that estimate is proportional to the same Σ_SFR and an assumed τ_mol_dep, it does not independently establish atomic dominance. This affects Conclusion 1 for the three dwarfs without CO coverage, while Conclusions 2–3 rest on the independent HI/SFR ratio, so the overall circularity is partial rather than pervasive.
Axiom & Free-Parameter Ledger
free parameters (3)
- M31 W1-subtraction slopes α (W4, FUV) =
0.135 (W4), 0.001 (FUV)
- Assumed molecular depletion time τ_mol_dep for CO-free galaxies =
0.7 Gyr (IC1613, NGC6822), 0.65 Gyr (WLM)
- Mass-to-light ratio Υ_3.4μm per galaxy =
0.31, 0.34, 0.31, 0.28, 0.28, 0.46 (IC10, IC1613, WLM, NGC6822, M33, M31)
axioms (7)
- domain assumption Optically thin 21-cm emission (A.1)
- domain assumption Lognormal distribution for f_corr (A.2)
- domain assumption CO-to-H2 conversion relation (Eq. A5)
- ad hoc to paper τ_mol_dep–M* relation extrapolation (A.4)
- domain assumption W1-to-stellar-mass conversion (A.6)
- domain assumption SFR tracer calibrations (A.5, A.7-A.11)
- domain assumption Fixed disk geometry for azimuthal averaging
read the original abstract
Resolving atomic gas at $\lesssim100$ pc physical resolution is currently feasible only for the very nearest galaxies. We combine 120 pc resolution HI data using the Karl G. Jansky Very Large Array (VLA) from the first data release of the Local Group L-Band Survey (LGLBS) with uniformly processed infrared, ultraviolet, H$\alpha$, and CO maps. We analyze the radial profiles of gas, stars, and recent star formation for six local galaxies: M31, M33, IC 10, IC 1613, NGC 6822, and WLM. Across the sample, atomic-gas disks are the most extended component, reaching $r_{\rm HI}=2.8$-$5.8$ kpc in the dwarfs and $13$-$26$ kpc in M33 and M31. Azimuthally averaged $\Sigma_{\rm HI}$ profiles are often flat or only slowly declining compared to the steeper decline of $\Sigma_{\star}$ and $\Sigma_\mathrm{SFR}$, with outer HI scale lengths of $l_{\rm HI}^{\rm outer}\approx1.0$-$8.5$ kpc. The mass-weighted HI surface densities on 120 pc scale generally exceed the azimuthally averaged values by 10-70%, showing that localized high-column-density atomic structures persist even where azimuthally averaged profiles appear smooth. Atomic-gas depletion times rise from $\tau_{\rm dep}^{\rm atom}\approx0.8$-17 Gyr within the stellar half-mass radius to 10s-100 Gyr in the outer disks. Molecular depletion times are shorter. We provide a public release of these multi-wavelength data, which provide a valuable reference for the LGLBS targets and also form the basis for a companion paper that relates star formation, gas phases, and midplane pressure on 120 pc scales.
Figures
Reference graph
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