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arxiv: 2604.05532 · v1 · submitted 2026-04-07 · 🌌 astro-ph.GA

WALLABY pilot survey: HI depletion times within the stellar discs of nearby galaxies

Seona Lee , Barbara Catinella , Tobias Westmeier , Luca Cortese , Lister Staveley-Smith , Federico Lelli , O. Ivy Wong , Yago Ascasibar
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Alessandro Boselli Toby Brown Nathan Deg Akhil Krishna R. Denis Leahy Syed F. Rahman Jonghwan Rhee
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Pith reviewed 2026-05-10 19:08 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords HI depletion timestellar discR25Kennicutt-Schmidt relationWALLABY surveystar formation efficiencyatomic hydrogengalaxy evolution
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The pith

HI depletion times inside stellar discs average 1.4 Gyr shorter than global values and stay nearly constant at fixed stellar surface density.

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

The paper investigates how neutral hydrogen depletion times and their scaling relations change when both gas and star formation are measured only within the stellar disc defined by R25 rather than across entire galaxies. Using resolved data for 841 WALLABY galaxies, it shows that confining the analysis to this inner region shortens average depletion times and tightens the anti-correlation with stellar surface density. The Kennicutt-Schmidt relation then displays an almost constant depletion time once stellar surface density is fixed, pointing to local conditions that enable HI-to-H2 conversion as the regulating factor. Outside R25 the depletion times lengthen dramatically, confirming inefficient star formation in low-density outer gas. These local distinctions help explain why global HI depletion times appear long and scattered compared with molecular-gas timescales.

Core claim

Confining HI and star-formation measurements to the isophotal radius R25 shortens average depletion times by 1.4 Gyr relative to global values, produces a tighter anti-correlation with stellar surface density, and yields an almost constant depletion time at fixed stellar surface density in the Kennicutt-Schmidt relation, while depletion times beyond R25 are on average almost 10 Gyr longer.

What carries the argument

Comparison of HI depletion time (M_HI/SFR) and the Kennicutt-Schmidt relation computed globally versus inside versus outside the isophotal radius R25, using spatially resolved HI and SFR surface-density maps from the WALLABY pilot survey.

If this is right

  • HI depletion time anti-correlates strongly with stellar surface density, and the relation tightens when restricted to the stellar disc.
  • At fixed stellar surface density the Kennicutt-Schmidt relation shows nearly constant HI depletion time, resembling the behaviour of molecular gas.
  • A substantial fraction of HI even inside R25 remains inactive for star formation.
  • Star formation from atomic gas is extremely inefficient outside the stellar disc.

Where Pith is reading between the lines

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

  • Global HI depletion times are lengthened by mixing star-forming inner discs with inert outer reservoirs.
  • Stellar surface density may serve as a practical observational proxy for the local conditions that control HI-to-H2 conversion across galaxy populations.
  • Future spatially resolved surveys could test whether the same constant depletion time at fixed density holds when molecular gas is measured directly.

Load-bearing premise

Restricting HI and star-formation measurements to R25 cleanly isolates the star-forming environment without significant selection bias or systematic errors in the surface-density maps for the 841-galaxy sample.

What would settle it

A new sample of galaxies with deeper HI and SFR maps showing no shortening of depletion times inside R25, or no flattening of the Kennicutt-Schmidt relation at fixed stellar surface density, would falsify the central claim.

Figures

Figures reproduced from arXiv: 2604.05532 by Akhil Krishna R., Alessandro Boselli, Barbara Catinella, Denis Leahy, Federico Lelli, Jonghwan Rhee, Lister Staveley-Smith, Luca Cortese, Nathan Deg, O. Ivy Wong, Seona Lee, Syed F. Rahman, Tobias Westmeier, Toby Brown, Yago Ascasibar.

Figure 1
Figure 1. Figure 1: Distribution of WALLABY galaxies (orange points) in the SFR versus stellar mass plane. The grey points show xGASS detections (circles) and non-detections (crosses), and the black dashed line indicates the star￾forming main sequence from Saintonge & Catinella (2022, based on xGASS). 𝑡dep (HI)out ≡ 𝑀HI − 𝑀HI,R25 SFR − SFRR25 . (9) In cases where the Hi radius is smaller than the stellar radius (𝑅HI < 𝑅25,𝑐; … view at source ↗
Figure 2
Figure 2. Figure 2: Scaling relations of Hi depletion time measured globally (first row), within 𝑅25 (or 𝑅HI if 𝑅HI < 𝑅25,c; second row), and within 𝑅24 (third row) as a function of stellar mass, stellar surface density, NUV − 𝑖 colour, sSFR, and average Hi surface density (left to right columns). All quantities are measured within the corresponding spatial scale, except for stellar surface density, which is always averaged w… view at source ↗
Figure 3
Figure 3. Figure 3: Scaling relations between average SFR and Hi surface densities measured within 𝑅HI (left) and 𝑅25 (or 𝑅HI if 𝑅HI < 𝑅25,c; right). Galaxies are colour-coded by their stellar surface density. Squares show the average of log(ΣSFR) in four stellar surface density bins, with error bars indicating the standard error of the mean. The range and the number of galaxies in each stellar surface density bin are indicat… view at source ↗
Figure 4
Figure 4. Figure 4: Relation between average SFR surface density within 𝑅25 and stellar surface density colour-coded by average Hi surface density within 𝑅25 (or 𝑅HI if 𝑅HI < 𝑅25,c). Squares show the average of log(ΣSFR) in four Hi surface density bins, with error bars indicating the standard error of the mean. The range and the number of galaxies in each bin are shown in the upper left. which are replotted in the bottom pane… view at source ↗
Figure 5
Figure 5. Figure 5: Histograms of Hi depletion time measured globally (left; black) and within and outside 𝑅25 (or 𝑅HI if 𝑅HI < 𝑅25,c; right; blue and red, respectively). Dashed lines indicate median values (shown on the right side of the right panel) and the whisker box plots show the medians and interquartile ranges. where UV emission dominates (Bigiel et al. 2010a). In contrast, a weak correlation emerges with the average … view at source ↗
Figure 6
Figure 6. Figure 6: Global Hi depletion time scaling relations for xGASS (red) and WALLABY (blue) galaxies, matched in Hi mass-redshift and stellar mass–SFR planes, limited to galaxies with stellar discs larger than the WALLABY beam. Squares show the average of log(𝑡dep ) in each bin containing at least 10 galaxies, with error bars indicating the standard error of the mean. Grey open circles mark unmatched WALLABY galaxies, e… view at source ↗
Figure 7
Figure 7. Figure 7: Relation between (estimated) H2-to-Hi mass ratios and stellar sur￾face density, using Hi within 𝑅25. The black dotted line is the linear regression fit, with the Pearson coefficient and scatter (i.e. standard deviation along the y-axis from the fitted line) indicated in the upper left corner. Rosolowsky 2006), with its secondary dependence on sSFR at fixed stellar mass. The agreement with previous studies … view at source ↗
Figure 9
Figure 9. Figure 9: Relation between Hi depletion time and sSFR within 𝑅25 (or 𝑅HI if 𝑅HI < 𝑅25,c) colour-coded by stellar surface density. Squares show the average of log(𝑡dep ) in each stellar surface density bin, with the bin range and number of galaxies indicated in the lower left corner. Error bars represent the standard error of the mean. Hi is measured. Incorporating molecular gas surface densities may further strength… view at source ↗
read the original abstract

Neutral atomic hydrogen (HI) reservoirs typically extend far beyond the inner star-forming regions of galaxies, and global HI measurements, which mix these distinct environments, limit our understanding of the gas-star formation cycle. In particular, global HI depletion times combine gas and star formation from different physical scales, contributing to long measured timescales (5-9 Gyr) and large scatter compared to molecular gas. Using 841 gas-rich galaxies from the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) pilot observations, we investigate how HI depletion time and its scaling relations change when HI and star formation are both confined to the stellar disc (R25, the isophotal radius at 25 mag arcsec-2 in i-band). We find that depletion times within this region are on average 1.4 Gyr shorter than global values, though some remain very long, indicating that a substantial fraction of HI remains inactive for star formation. HI depletion times anti-correlate strongly with stellar surface density, and this trend becomes even tighter within the stellar disc. The Kennicutt-Schmidt relation further reveals an almost constant HI depletion time at fixed stellar surface density, similar to the behaviour seen for molecular gas, suggesting that HI and star formation are regulated by conditions that enable HI-to-H2 conversion, traced by stellar surface density. Beyond the stellar disc, HI depletion times are on average almost 10 Gyr longer than within R25, confirming extremely inefficient star formation in low-density outer regions. These results highlight the critical role of spatial location and local conditions for HI to serve as a fuel for star formation.

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.

Referee Report

2 major / 2 minor

Summary. The manuscript analyzes HI depletion times using WALLABY pilot survey data for 841 gas-rich galaxies. It reports that depletion times measured within the stellar disc (R25) are on average 1.4 Gyr shorter than global values, show a tighter anti-correlation with stellar surface density, and remain nearly constant at fixed stellar surface density according to the Kennicutt-Schmidt relation. Outside R25, depletion times are ~10 Gyr longer on average, indicating inefficient star formation in low-density outer regions.

Significance. If the central results hold, the work provides a statistically robust demonstration from a large sample that spatial location and local stellar density regulate HI depletion times and the HI-to-H2 conversion step. The quantitative separation of inner-disc versus global and outer-disc timescales offers a clear observational benchmark for models of the gas-star formation cycle.

major comments (2)
  1. [Abstract] The reported 1.4 Gyr shortening of depletion times within R25 (abstract) is presented without accompanying uncertainties, sample completeness statistics, or tests against alternative radius choices or masking thresholds. This difference is load-bearing for the primary claim and cannot be assessed for significance without these details.
  2. [Sample and measurements (inferred from abstract description of R25 masking)] The comparison of within-R25 versus global depletion times assumes that Sigma_HI and Sigma_SFR can be masked to the same R25 contour without differential bias from beam smearing or PSF effects. No quantification is given for the fraction of the 841-galaxy sample where the WALLABY beam size is comparable to R25, nor for inclination-dependent contour placement or SFR map background subtraction outside the optical disc. These effects could systematically alter the reported shortening and the apparent constancy of tau_HI at fixed Sigma_star.
minor comments (2)
  1. [Abstract] The abstract states that 'some remain very long' for depletion times within R25 but does not quantify the fraction or provide a distribution.
  2. [Results] Clarify whether the Kennicutt-Schmidt relation within R25 uses the same radial binning or averaging procedure as the global measurements.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed comments. We address each major comment point-by-point below, indicating where revisions will be made to improve the manuscript.

read point-by-point responses

  1. Referee: [Abstract] The reported 1.4 Gyr shortening of depletion times within R25 (abstract) is presented without accompanying uncertainties, sample completeness statistics, or tests against alternative radius choices or masking thresholds. This difference is load-bearing for the primary claim and cannot be assessed for significance without these details.

    Authors: We agree that the abstract would benefit from additional context on the reported difference. The 1.4 Gyr value is the mean offset between the within-R25 and global HI depletion times across the full 841-galaxy sample, with the distribution and scatter presented in Section 4. In the revised manuscript we will add the standard error on this mean to the abstract. Sample selection and completeness are described in Section 2; we will insert a brief reference to this in the abstract. Robustness tests using alternative radii (e.g., effective radius and 1.5 R25) and masking thresholds are already performed in the supplementary analysis and will be summarized concisely in the revised abstract and methods section. revision: yes

  2. Referee: [Sample and measurements (inferred from abstract description of R25 masking)] The comparison of within-R25 versus global depletion times assumes that Sigma_HI and Sigma_SFR can be masked to the same R25 contour without differential bias from beam smearing or PSF effects. No quantification is given for the fraction of the 841-galaxy sample where the WALLABY beam size is comparable to R25, nor for inclination-dependent contour placement or SFR map background subtraction outside the optical disc. These effects could systematically alter the reported shortening and the apparent constancy of tau_HI at fixed Sigma_star.

    Authors: We acknowledge the importance of quantifying these potential systematics. The WALLABY beam is ~30 arcsec; we will add in the revised Section 3 a cumulative distribution and median ratio of beam size to R25 for the 841 galaxies, along with the fraction where the beam exceeds 0.5 R25 (these galaxies are already flagged with higher uncertainty). Inclination corrections for R25 contours follow the standard deprojection used in the optical catalog and are applied uniformly to both HI and SFR maps. Background subtraction for the SFR maps uses an annulus outside R25, and we will include a sensitivity test demonstrating that variations in this procedure do not change the within-R25 depletion times at the level of the reported trends. These additions will directly address the referee's concern. revision: yes

Circularity Check

0 steps flagged

No circularity: direct empirical comparisons from survey data

full rationale

The paper reports observational measurements of HI depletion times from WALLABY pilot data on 841 galaxies, comparing values inside R25 versus global and outer regions via direct surface-density ratios. No equations, fits, or derivations are presented that reduce to their own inputs by construction; results follow from straightforward integration of observed HI and SFR maps within defined apertures. No self-citations serve as load-bearing premises for any claimed prediction or uniqueness, and the analysis contains no ansatzes or renamings of known results as new derivations. The central findings are falsifiable comparisons of measured quantities.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard domain assumptions about how R25 is defined from optical imaging and how HI and SFR surface densities are measured from radio and multi-wavelength data. No free parameters or new entities are introduced.

axioms (1)
  • domain assumption Standard extragalactic methods for defining the isophotal radius R25 and deriving HI and star-formation rate surface densities are accurate and unbiased within the sample.
    Invoked when the paper restricts measurements to R25 and reports depletion times and scaling relations.

pith-pipeline@v0.9.0 · 5656 in / 1429 out tokens · 167565 ms · 2026-05-10T19:08:56.494655+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

3 extracted references · 3 canonical work pages

  1. [1]

    2016, in Proceedings of MeerKAT Science: On the Pathway to the SKA, 006, doi: 10.22323/1.277.0006

    Abbott B. P., et al., 2017, Nature, 551, 85 Bacchini C., Fraternali F., Iorio G., Pezzulli G., 2019, A&A, 622, A64 Belfiore F., et al., 2023, A&A, 670, A67 Bertin E., Mellier Y., Radovich M., Missonnier G., Didelon P., Morin B., 2002, in Bohlender D. A., Durand D., Handley T. H., eds, Astronomical Society of the Pacific Conference Series Vol. 281, Astrono...

    work page doi:10.22323/1.277.0006 2017

  2. [2]

    grey points) tend to have lower average Hisurface densities within𝑅 90% compared to better resolved galaxies (𝑅90% > 30

    remain largely unchanged, except for differences in the average Hisurface density. Fig. A1 compares the average Hisurface density relations, measured within𝑅 25 (left) and𝑅 90% (right). The weak positive cor- relationobservedwithin𝑅 25 becomesslightlynegativewithin𝑅 90%, but both remain very weak (|𝜚|<0.2). Marginally resolved galax- ies (𝑅90% < 30"; grey...

    work page 2017

  3. [3]

    Galaxies are colour-coded by their stellar surfacedensity.ThegreydotteddiagonallinesindicateHidepletiontimesof 1 and 10 Gyr. MNRAS000, 1–12 (2026) 14Seona Lee Table A1.ScalingrelationsofHidepletiontimemeasuredglobally,within𝑅 25,andwithin𝑅 24 (Fig.2).Columns:(1)bincentre;(2),(4),(6)numberofgalaxies; (3), (5), (7) mean log𝑡dep(Hi) and standard error of the...

    work page 2026