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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 →

arxiv 2607.21841 v1 pith:HFNM5N7N submitted 2026-07-23 astro-ph.GA

A Multiwavelength Inventory for the Local Group L-band Survey I: Atlas and Radial Profiles of Local Group Galaxies

classification astro-ph.GA
keywords Local Group galaxiesatomic hydrogenstar formation depletion timeHI-to-H2 transitionradial profilesdwarf galaxiesgalaxy evolutioninterstellar medium
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

This paper combines 120-pc-resolution HI maps from the Local Group L-Band Survey with uniformly processed infrared, ultraviolet, Hα, and CO data to build the first coherent multiwavelength inventory of six nearby galaxies: M31, M33, IC 10, IC 1613, NGC 6822, and WLM. It establishes that atomic gas is the most extended baryonic component, often flatter than the stellar and star-formation profiles, and that outside the stellar effective radius the atomic-gas depletion time always exceeds 10 Gyr. The authors argue that, across this whole sample, the balance between atomic and molecular gas — not the efficiency of star formation within molecular clouds — is the factor that regulates where and how fast stars form. If correct, this refocuses galaxy evolution models on the HI-to-H2 transition as the key bottleneck, and it provides a benchmark dataset for studying that transition at physical scales below 100 pc.

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.

Watch this falsifier — get emailed when new claim-graph text bears on it.

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

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

  • 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.

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

Referee Report

2 major / 6 minor

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)
  1. [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.
  2. [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)
  1. [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.
  2. [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.
  3. [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.
  4. [Table 2] The header 'Hirot.' appears to be a garbled abbreviation; it should read 'HI rotation curve' for clarity.
  5. [References] The Koch et al. (2025) reference has a malformed author list ('Koch, E., LGLBS, C., LGLBS, C., ...'). Please fix it.
  6. [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

1 steps flagged

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
  1. 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

3 free parameters · 7 axioms · 0 invented entities

The paper introduces no new physical entities; its free parameters are calibration values and corrections. The main burdens are the adopted external calibrations, two fitted M31 correction slopes, and the assumed molecular depletion times for CO-free dwarfs.

free parameters (3)
  • M31 W1-subtraction slopes α (W4, FUV) = 0.135 (W4), 0.001 (FUV)
    Fit to inner M31 region to remove old-stellar IR/UV contamination (Appendix B); enters corrected M31 SFR maps and hence M31 depletion times.
  • Assumed molecular depletion time τ_mol_dep for CO-free galaxies = 0.7 Gyr (IC1613, NGC6822), 0.65 Gyr (WLM)
    Extrapolated from the Sun et al. (2025) τ_mol_dep–M* relation (A.4); used to create expected Σ_mol profiles and Table 5 M_mol for galaxies without useful CO. These are not measured values.
  • 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)
    Adopted from SED-based calibration (A.6; Leroy et al. 2019) and used to convert W1 light to Σ*. A constant M/L does not change profile shapes, but the absolute stellar masses and stellar scale lengths are affected.
axioms (7)
  • domain assumption Optically thin 21-cm emission (A.1)
    Used to convert I_21cm to Σ_HI; if optical depth is significant, atomic gas masses/surface densities are underestimated.
  • domain assumption Lognormal distribution for f_corr (A.2)
    The bias correction for mass-weighted Σ_atom assumes the 120 pc column density distribution is lognormal within each ring; non-lognormal distributions would bias clumping factors.
  • domain assumption CO-to-H2 conversion relation (Eq. A5)
    α_CO scaling with metallicity and Σ* from Schinnerer & Leroy (2024) is adopted as an external calibration; affects Σ_mol for IC10, M33, M31.
  • ad hoc to paper τ_mol_dep–M* relation extrapolation (A.4)
    Sun et al. (2025) relation derived for M*=1e9.5–11 M⊙ is extrapolated down to ~1e7.5–8 M⊙ for WLM/IC1613/NGC6822; this is the basis for expected molecular gas estimates.
  • domain assumption W1-to-stellar-mass conversion (A.6)
    Υ_3.4μm from Salim/Leroy SED calibrations is used; the reported stellar masses depend on this.
  • domain assumption SFR tracer calibrations (A.5, A.7-A.11)
    FUV, W4, Hα, and combined SFR conversions from Leroy et al. (2019), Murphy et al. (2011), Calzetti et al. (2007), Belfiore et al. (2023) are treated as accurate.
  • domain assumption Fixed disk geometry for azimuthal averaging
    Profiles assume galaxies are thin inclined disks with LEDA inclinations/PAs; warps or asymmetric HI (e.g., NGC 6822) bias axisymmetric profiles.

pith-pipeline@v1.3.0-alltime-deepseek · 36312 in / 17287 out tokens · 159429 ms · 2026-08-01T06:32:19.185168+00:00 · methodology

0 comments
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

Figures reproduced from arXiv: 2607.21841 by Adam K. Leroy, Adam Smercina, Alberto D. Bolatto, Amanda A. Kepley, Chang-Goo Kim, Christina W. Lindberg, Cosima Eibensteiner, Elizabeth Tarantino, Eric W. Koch, Erik Roslowsky, Eve C. Ostriker, Fabian Walter, Jiayi Sun, Julianne J. Dalcanton, J\"urgen Ott, Laura B. Chomiuk, Michael P. Busch, Nickolas Pingel, Ryan Chown, Sne\v{z}ana Stanimirovi\'c, Sumit K. Sarbadhicary, Thomas G. Williams, Tobin M. Wainer, Vicente Villanueva.

Figure 1
Figure 1. Figure 1: Atlas of gas, stars and star formation for IC 10. The upper two maps show field of views matched to the H i map. The lower four panels show zoomed in versions to better illustrate the connection to the other tracers. The white dashed circle shows the rHI taken from [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Azimuthally-averaged surface density profiles for IC 10. The x-axes show galactocentric radius in physical units and normalized to the optical isophotal radius (r25). Upper left: Gas surface densities. Blue colors show the atomic gas surface density Σatom (Equation A1) including the 16-84% range as a shaded blue region. The vertical blue dashed line denotes rHI, the radius where Σatom = 1 M⊙ pc−2 . In ligh… view at source ↗
Figure 3
Figure 3. Figure 3: Atlas of gas, stars and star formation, same as [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Radial profiles same as [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Atlas of gas, stars and star formation, same as [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Radial profiles same as [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Atlas of gas, stars and star formation, same as [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Radial profiles same as [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Atlas of gas, stars and star formation, same as [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Radial profiles same as [PITH_FULL_IMAGE:figures/full_fig_p015_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Atlas of gas, stars and star formation, same as [PITH_FULL_IMAGE:figures/full_fig_p016_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Radial profiles same as [PITH_FULL_IMAGE:figures/full_fig_p017_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Correction for IR cirrus and stellar contamination in M31. The left panel shows the W4 map corrected by scaling and subtracting the W1 map. The right panel shows the uncorrected W4 map. The correction removes a large amount of emission not associated with recent star formation from the central region of M31 but does not significantly affect the disk (see [PITH_FULL_IMAGE:figures/full_fig_p029_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Left: Curves show the W4 (22 µm) annular profiles for M31: uncorrected (blue) and after subtracting the W1-based contaminant estimate (orange). The W1-based subtraction suppresses central emission unassociated with star formation while preserving the star-forming ring/outer disk. Right: The same for FUV profiles. See Appendix B [PITH_FULL_IMAGE:figures/full_fig_p030_14.png] view at source ↗

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