GASP. XX. From the loose spatially-resolved to the tight global SFR-Mass relation in local spiral galaxies
Pith reviewed 2026-05-25 11:40 UTC · model grok-4.3
The pith
The tight global SFR-M_star relation in local spirals is driven by the size-mass relation, even as the resolved ΣSFR-Σ_star relation stays loose.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Considering all galaxies together the correlation between ΣSFR and Σ_star is broad, yet each galaxy follows its own distinct relation whose scatter is set mainly by off-center star-forming knots; the surface gas density does not tighten the relation. Mean ΣSFR and Σ_star still vary by at most 0.7 dex across the sample. The global SFR-M_star relation emerges because the size-mass relation maps total stellar mass to total size, converting the loose local measurements into a tight integrated scaling.
What carries the argument
The size-mass relation, which connects a galaxy's total stellar mass to its effective radius and thereby averages local surface-density variations into a tight global SFR-M_star correlation.
If this is right
- The scatter in the resolved relation is produced mainly by bright off-center star-forming knots whose own ΣSFR-Σ_star slopes are broader than the diffuse component.
- The ΣSFR-Σ_tot gas relation is as broad as the stellar one, so gas surface density is not the main driver of the observed trend.
- Excluding the galaxy outskirts steepens the resolved relation and shifts it toward higher Σ_star values, with the change larger in more massive systems.
- Despite large galaxy-to-galaxy differences, average surface densities across the sample span no more than 0.7 dex.
Where Pith is reading between the lines
- Global scaling relations in other galaxy properties may likewise be shaped more by the size-mass correlation than by the details of local physics.
- High-redshift surveys that measure both resolved star formation and galaxy sizes could test whether the same averaging mechanism operates when the size-mass relation itself evolves.
- Semi-analytic or simulation models must reproduce the observed size-mass relation to the same precision as the global SFR-M_star relation if the two are causally linked in this way.
Load-bearing premise
The 30 galaxies are representative of the undisturbed late-type population and their outer Hα measurements accurately reflect star formation without major systematic errors.
What would settle it
A sample of galaxies that obey a tight global SFR-M_star relation even after the size-mass correlation is removed or absent would falsify the claim that the size-mass relation is the driver.
read the original abstract
Exploiting the sample of 30 local star-forming, undisturbed late-type galaxies in different environments drawn from the GAs Stripping Phenomena in galaxies with MUSE (GASP), we investigate the spatially resolved Star Formation Rate-Mass ({\Sigma}SFR-{\Sigma}_star) relation. Our analysis includes also the galaxy outskirts (up to >4 effective radii, re), a regime poorly explored by other Integral Field Spectrograph surveys. Our observational strategy allows us to detect H{\alpha} out to more than 2.7re for 75% of the sample. Considering all galaxies together, the correlation between the {\Sigma}SFR and {\Sigma}_star is quite broad, with a scatter of 0.3 dex. It gets steeper and shifts to higher {\Sigma}_star values when external spaxels are excluded and moving from less to more massive galaxies. The broadness of the overall relation suggests galaxy-by-galaxy variations. Indeed, each object is characterized by a distinct {\Sigma}SFR-{\Sigma}_star relation and in some cases the correlation is very loose. The scatter of the relation mainly arises from the existence of bright off-center star-forming knots whose {\Sigma}SFR-{\Sigma}_star relation is systematically broader than that of the diffuse component. The {\Sigma}SFR-{\Sigma}tot gas (total gas surface density) relation is as broad as the {\Sigma}SFR-{\Sigma}_star relation, indicating that the surface gas density is not a primary driver of the relation. Even though a large galaxy-by-galaxy variation exists, mean {\Sigma}SFR and {\Sigma}_star values vary of at most 0.7 dex across galaxies. We investigate the relationship between the local and global SFR-M_star relation, finding that the latter is driven by the existence of the size-mass relation.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper uses MUSE integral-field spectroscopy from the GASP survey to examine the spatially resolved ΣSFR–Σ⋆ relation in 30 undisturbed local late-type galaxies, extending measurements to >4 Re. It reports a broad overall correlation (0.3 dex scatter) that steepens and shifts with galaxy mass when outer spaxels are excluded, with substantial galaxy-to-galaxy variation driven by off-center star-forming knots; the ΣSFR–Σgas relation is comparably broad, implying gas density is not the primary driver. Mean surface densities vary by ≤0.7 dex across the sample. The central conclusion is that the tight global SFR–M⋆ relation is driven by the size–mass relation.
Significance. If the interpretation is substantiated, the work supplies a concrete observational link between loose local scaling relations and the tightness of the global star-formation main sequence, emphasizing the role of galaxy size in averaging local variations. The extension to large radii and the separation of knot versus diffuse components are useful additions to the IFU literature on resolved star formation.
major comments (2)
- [Abstract] Abstract and concluding discussion: the claim that the global SFR–M⋆ relation “is driven by the existence of the size-mass relation” is presented as a finding but is not accompanied by a quantitative forward model, integration over galaxy areas, or direct comparison of predicted versus observed global slope and scatter. Without such a demonstration the statement remains an interpretation rather than a tested result and is load-bearing for the paper’s central claim.
- [Methods / Results] Methods and results sections: the manuscript provides no tabulated data, no explicit error budget for the derived surface densities, and no quantitative assessment of selection or completeness biases in the outer-disk Hα detections (75 % of galaxies detected beyond 2.7 Re). These omissions prevent independent verification of the reported scatters and mean offsets.
minor comments (2)
- [Abstract] Abstract, line “mean ΣSFR and Σ⋆ values vary of at most 0.7 dex”: correct preposition to “vary by at most”.
- [Results] The distinction between “bright off-center star-forming knots” and the “diffuse component” is introduced without a quantitative definition or surface-brightness threshold; this should be stated explicitly when the two populations are compared.
Simulated Author's Rebuttal
We thank the referee for their thoughtful and constructive comments on our manuscript. We address each of the major comments below and will revise the paper accordingly.
read point-by-point responses
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Referee: [Abstract] Abstract and concluding discussion: the claim that the global SFR–M⋆ relation “is driven by the existence of the size-mass relation” is presented as a finding but is not accompanied by a quantitative forward model, integration over galaxy areas, or direct comparison of predicted versus observed global slope and scatter. Without such a demonstration the statement remains an interpretation rather than a tested result and is load-bearing for the paper’s central claim.
Authors: We agree with the referee that our interpretation would benefit from a more quantitative demonstration. While our analysis shows that local relations have significant scatter but mean values are similar across galaxies, and we link this to the size-mass relation, we did not perform an explicit forward modeling. In the revised manuscript, we will add a quantitative section that integrates the observed local ΣSFR-Σ⋆ relations over galaxy disks using the size-mass relation to predict the global SFR-M⋆ slope and scatter, and compare these predictions to the observed global relation. revision: yes
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Referee: [Methods / Results] Methods and results sections: the manuscript provides no tabulated data, no explicit error budget for the derived surface densities, and no quantitative assessment of selection or completeness biases in the outer-disk Hα detections (75 % of galaxies detected beyond 2.7 Re). These omissions prevent independent verification of the reported scatters and mean offsets.
Authors: We will address these points in the revision. An explicit error budget for the surface density measurements will be added to the Methods section. We will also include a quantitative assessment of the completeness and selection biases for the outer-disk Hα detections, including an analysis of the detection limits and potential missed flux. Regarding tabulated data, we will add a summary table with per-galaxy statistics (means, scatters, etc.) and make the full spaxel catalog available through a public data repository to allow independent verification. revision: yes
Circularity Check
No circularity: purely observational empirical relations from MUSE data
full rationale
The paper presents an observational analysis of spatially resolved ΣSFR-Σ_star relations in 30 galaxies using MUSE Hα data. The central statement that the global SFR-M_star relation is 'driven by' the size-mass relation is presented as an interpretive finding from comparing local scatters (0.3 dex) and mean variations (≤0.7 dex) to global tightness, without any equations, fitted parameters, forward models, or self-referential definitions that reduce the result to its inputs by construction. No self-citations are invoked for uniqueness theorems, ansatzes, or load-bearing premises. The work is self-contained as direct data reduction and pattern reporting; no step matches the enumerated circularity patterns.
discussion (0)
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