The main thing to know is that stars, dust, neutral gas, and supermassive black holes inside the optical radii of galaxies add up to only about 5% of the total baryons out to redshift 3. The rest must sit as ionized material in the CGM and IGM. They reach this by running ProSpect SED fits on roughly 800,000 galaxies from GAMA and DEVILS, deriving stellar and dust mass densities, then scaling dust to neutral gas with a metallicity-dependent ratio. Prior SMBH densities are added in to close the account. The dust mass history tracks the star-formation rate but falls off more slowly, which is a straightforward observational result worth noting. The large sample and joint modeling of multiple components give a self-consistent census that was not already in the literature. That synthesis is the real contribution here. The neutral-gas step is the soft spot. The dust-scaled density sits 0.7 dex below 21 cm surveys, which the authors attribute to dust being more centrally concentrated than extended HI. If the scaling undercounts gas even after restricting to optical radii, or if metallicity uncertainties are bigger than assumed, the confined baryon total rises and the 95% outside claim softens. The abstract shows no error budgets or sensitivity tests on this conversion, so the central number is harder to trust than the stellar and dust pieces. This paper is for people working on the missing-baryons problem or on how gas cycles through galaxies. A reader who needs benchmark cosmic densities for stars, dust, and gas at 0
Referee Report
1 major / 2 minor
Summary. The manuscript uses ProSpect SED modelling of ~800,000 galaxies from the GAMA and DEVILS surveys to derive the cosmic stellar, dust and neutral gas mass histories at 0 < z ≲ 3. Neutral gas is obtained by scaling dust masses with a metallicity-dependent dust-to-gas ratio; the resulting density lies ~0.7 dex below 21 cm measurements, which the authors attribute to spatial-scale differences. Incorporating prior SMBH mass densities, the paper concludes that stars, dust, neutral gas and SMBHs confined within galaxy optical radii account for only ~5% of the total baryon density, implying that the remaining ~95% must be ionised and dispersed throughout the ISM, CGM and IGM.
Significance. If the central 5% fraction is robust, the work supplies a valuable, internally consistent benchmark for the baryon budget inside galaxies across cosmic time. It strengthens the observational case that the majority of baryons reside outside optical radii and provides a reference point for CGM/IGM studies and simulations. The use of a single large, homogeneous dataset and modelling pipeline for the stellar, dust and gas components is a clear methodological strength.
major comments (1)
- Neutral gas estimation section: Neutral gas is derived by scaling dust masses (from ProSpect) with a metallicity-dependent dust-to-gas ratio, producing a cosmic density ~0.7 dex below 21 cm surveys. The manuscript attributes the offset to differing spatial scales but provides no quantitative estimate of the neutral-gas fraction still contained within the optical radii nor any sensitivity tests to metallicity assumptions. Because neutral gas supplies the largest adjustable term in the confined-baryon sum, this unquantified systematic directly affects the robustness of the ~5% total and the inference that ~95% of baryons lie outside galaxies.
minor comments (2)
- Abstract: The abstract states the 0.7 dex offset but supplies neither error budgets nor sensitivity tests for the dust-to-gas scaling relation used to obtain the neutral-gas density.
- Results section: Figures showing mass-density histories should explicitly label the optical-radius restriction and include direct overlays of the 21 cm comparison data with uncertainties.
Simulated Author's Rebuttal
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We thank the referee for their constructive report and for recognising the potential value of this work as an internally consistent benchmark for the baryon budget. We address the single major comment below and indicate where revisions will be made.
read point-by-point responses
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Authors: We agree that additional quantification would strengthen the presentation. The 0.7 dex offset is attributed to spatial-scale differences because the dust masses (and thus the scaled neutral-gas masses) are derived from SED modelling that is effectively limited to the regions contributing to the observed optical/near-IR light, i.e., within the optical radii. In contrast, 21 cm surveys integrate over more extended HI disks and, in some cases, low-column-density gas that may lie in the CGM. While our dataset does not contain resolved HI maps that would allow a direct measurement of the intra-optical-radius HI fraction, we can place the result in context by noting that local-universe studies (e.g., THINGS and HALOGAS) typically find 30–60 % of the total HI mass within the optical radius for galaxies of similar stellar mass. To address the lack of sensitivity tests, we have now performed additional runs in which the dust-to-gas ratio is varied by ±0.25 dex around the fiducial metallicity-dependent relation (consistent with the observed scatter in the mass–metallicity relation and our own metallicity uncertainties). These tests change the neutral-gas mass density by at most 0.2 dex and leave the total confined-baryon fraction between 4 % and 6 %. We will incorporate both the literature context for the spatial-scale argument and the new sensitivity tests into a revised section of the manuscript.
revision: partial
Circularity Check
1 steps flagged
SMBH mass density folded in from prior work using similar data/methods creates partial self-citation dependency in the 5% baryon census
specific steps
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self citation load bearing
[Abstract]
"Folding in measurements of the supermassive black hole mass density obtained previously with similar data and methods, we present a self-consistent census of the baryons confined to galaxies. Stars, neutral gas, SMBHs and dust contained within the optical radii of galaxies account for ≈ 5 per cent of the baryons."
The headline 5% baryon fraction within optical radii is the sum of four terms; the SMBH term is imported from earlier work that employed comparable surveys and modelling pipelines. While the stellar, dust and neutral-gas terms are freshly computed here, the inclusion of the SMBH component means the final numerical result is not fully independent of prior author-group outputs.
full rationale
The paper independently derives stellar, dust and neutral-gas densities from GAMA/DEVILS ProSpect SED fits at 0<z≲3. Neutral gas is obtained by scaling dust masses with a metallicity-dependent dust-to-gas ratio (an external assumption, not a fit to the target baryon fraction). The only load-bearing external input is the SMBH mass density, explicitly described as 'obtained previously with similar data and methods'. This introduces a self-citation element into the final 5% total but does not render the central claim equivalent to its inputs by construction; the stellar/dust/gas terms remain independently measured. No self-definitional loops, fitted inputs renamed as predictions, or ansatz smuggling are present. Hence a modest circularity score of 4 is appropriate.
Axiom & Free-Parameter Ledger
1 free parameters ·
1 axioms ·
0 invented entities
The central claim rests on the assumption that the dust-to-gas scaling relation calibrated at low redshift remains valid across the probed redshift range and that the optical-radius aperture captures the relevant baryonic mass. No new particles or forces are introduced.
free parameters (1)
- metallicity-dependent dust-to-gas ratio
Used to convert dust mass to neutral gas mass; the functional form and any normalization constants are not specified in the abstract.
axioms (1)
- domain assumption Total cosmic baryon density is known from standard cosmology (CMB or Big Bang nucleosynthesis).
Required to convert absolute mass densities into a percentage of all baryons.
pith-pipeline@v0.9.0 ·
5835 in / 1389 out tokens ·
31444 ms ·
2026-05-21T16:54:28.584657+00:00
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