REVIEW 2 major objections 5 minor 3 cited by
The evolution of the sizes and angular momentum content of galaxies in the COLIBRE simulations
T0 review · 2 major / 5 minor · reviewed 2026-08-02 · deepseek-v4-flash
Pith's one-line read The COLIBRE simulations reproduce the observed galaxy size–mass and angular-momentum–mass relations across cosmic time, for discs and spheroids alike, with a high-redshift offset attributed to dust.
desk verdict Solid first systematic COLIBRE size/angular-momentum validation paper: transparent about calibration, strong on independent tests, but the high-z dust explanation is a real soft spot that needs to be shown, not just cited. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the COLIBRE suite of cosmological hydrodynamical simulations, which uses a four-to-one dark-matter-to-baryon particle ratio, cooling below 10^4 K, and stellar and AGN feedback calibrated to observed z=0 galaxy masses and size–mass relations. The quantities carrying the argument are the stellar specific angular momentum (angular momentum per unit stellar mass), the three-dimensional and projected stellar half-mass radii, wavelength-dependent half-light radii, and the R1 radius defined by a stellar surface density threshold of 1 solar mass per square parsec. The paper builds its case by comparing simulation to observation across all of these definitions, across morphologi
What would settle it
Measure half-light radii from dust-attenuated mock images of the simulated galaxies, using the dust masses COLIBRE already tracks, and compare them to the same observed samples at z≈0 and z≈2–3. If the simulated u-band and near-infrared sizes do not grow by roughly 0.1–0.4 dex at the relevant masses, the dust explanation for the high-redshift discrepancy collapses.
Extended reading notes
Core claim
The central claim, on the paper's own terms, is that COLIBRE—a cosmological hydrodynamical simulation with gas cooling below 10^4 K and feedback calibrated to observed z=0 stellar masses and sizes—simultaneously reproduces the observed size–mass and specific angular momentum–mass relations for central galaxies over a wide mass range and across redshift. The agreement holds for multiple, physically distinct size definitions: three-dimensional and projected half-mass radii, rest-frame u-, r-, and z-band half-light radii, the radius enclosing 90 per cent of the light, the R1 radius where stellar surface density falls below 1 solar mass per square parsec, and baryonic half-mass radii. The simula
Load-bearing premise
The claim rests on the assumption that dust—neglected in the simulated size measurements but present in observed sizes—neither creates the apparent agreement at z=0 nor hides a genuine structural failure at z≳2.
Editorial extensions
If this is right
- If the central claim is right, galaxy sizes and spins in the model are set by the same feedback-regulated processes that reproduce the stellar mass function, so structure does not require independent fine-tuning over most of the sampled range.
- The parallel star-forming and passive sequences in the angular momentum–mass plane imply that disc–spheroid differences in size and spin are a single predicted outcome of the model, not separate adjustments.
- If dust explains the high-redshift offset, size evolution inferred from rest-frame optical or UV observations at z≳2 includes a dust contribution and is not a pure measure of structural change.
- The agreement for star-forming galaxies out to z≈1.5 gives a quantitative constraint on how much halo angular momentum galaxies retain at early times, which is central to disc formation theory.
- The z≈0 agreement across many size definitions makes the simulation a practical tool for interpreting observed sizes, including low-surface-brightness measurements at large radii.
Reading between the lines
- If dust really explains the z≳2 gap, then dust-free rest-frame near-infrared sizes should agree with the simulations better than optical sizes do; this is directly testable with existing multi-band samples.
- Since COLIBRE already tracks dust, the authors could turn the suspected explanation into a prediction by measuring half-light radii from mock images that include dust attenuation; the paper leaves this as an explicit next step.
- The strong aperture dependence of specific angular momentum for massive galaxies suggests that observational estimates built from extrapolated rotation curves may systematically miss halo-scale angular momentum, which matters when comparing the high-mass end of the j–M plane.
- A natural extension is to apply the same comparison to satellites and to morphologies defined by bulge-to-total ratios rather than kinematic disc fractions, which would test whether the structural realism extends beyond central galaxies.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper analyzes galaxy sizes and specific angular momentum in the COLIBRE cosmological hydrodynamical simulations across three resolution levels, comparing with observed scaling relations from z=0 to z=4. At z=0, COLIBRE is reported to reproduce the observed size–mass relation for several definitions—2D/3D stellar half-mass radii, Sérsic-fit mass- and r-band radii, u/z-band half-light and 90%-light radii, the R1 surface-density radius, and baryonic half-mass radii—including splits by morphology and star-formation activity. The stellar and baryonic specific angular momentum–mass relations also match local observations, and the stellar j*-M* relation matches star-forming galaxies at z≈0.5–1.5. At z≳2, intermediate-mass COLIBRE galaxies are 0.1–0.5 dex smaller than observed, and the star-forming/passive size separation is muted; the paper attributes this to neglected dust attenuation, supported only by an unpublished preliminary test. Section 2.4 is transparent that the z=0 half-mass radius relation used for calibration is not an independent test.
Significance. If the conclusions were fully supported, this would be a valuable validation of a modern simulation suite: the multi-resolution COLIBRE set, the breadth of size definitions (R1, baryonic radii, multi-band light radii), and the inclusion of j*-M* and baryonic j-M relations provide genuinely independent constraints beyond the calibrated z=0 Rstar50 relation. The appendices on convergence, satellites, aperture effects, and stellar-mass scatter are useful and strengthen the paper. Public code and data availability are additional assets. The main limitation is that the z>2 size discrepancy—which the headline 'across cosmic time' claim depends on—is reconciled by an untested dust argument rather than a forward-modelled mock-observation analysis. This is fixable by adding the mock analysis or by restricting the success claim to z≲1.5–2.
major comments (2)
- [§3.2, Fig. 10; also Fig. 6] Fig. 10 shows that at 2.0<z<4.0 and 10^9.5≲M*/M⊙≲10^11, the simulated R_1µm,50 values are 0.1–0.5 dex smaller than Song et al. (2025). The only quantitative reconciliation is the one-sentence statement in §3.2 that 'preliminary tests indicate that accounting for dust attenuation increases the apparent sizes... by ~0.1–0.4 dex at z=3.' This test is not shown and is not a full mock: no PSF convolution, GALFIT measurement, or Song et al. resolution/quality cuts are applied. The same ad hoc dust explanation is used for the low R_u,90 at z=0. Because the claim of a 'self-consistent and observationally successful framework' spans z=0–4, this untested premise is load-bearing. I request a full forward-modelled dust comparison (or at least a described quantitative test), or a revision that states agreement at z≲1.5–2 and reports z>2 as an open tension.
- [§3.2, Fig. 9; §4 item (v)] The reduced separation between star-forming and passive size–mass relations at z≳0.75 (Fig. 9) is a second discrepancy relevant to the morphology-evolution claim. For the Song et al. comparison the paper states 'we do not show these results explicitly', and then appeals again to dust. The statement in §3.2 that differences may reflect dust attenuation is a hypothesis, not a demonstrated result. Please quantify the star-forming/passive split (even in an appendix) or temper the claim that COLIBRE reproduces the observed morphology segregation at high redshift.
minor comments (5)
- [Fig. 13] The in-panel labels and the text/reference list disagree: 'Swinbank et al. (2022)' appears in a panel while the text and references use Swinbank et al. (2017); similarly, 'Gillman et al. (2017)' conflicts with Gillman et al. (2020). Please correct.
- [Throughout] Typos: 'correlats' in §1, 'defintion' in the Fig. 2 caption, 'fidcuial' in Appendix C, 'the the' in §3.5, and 'V ogelsberger' in the reference list.
- [Fig. 10 caption] The caption says sizes are for 'random orientations', whereas §2.3 defines 2D sizes as averages over three orthogonal projections. Use consistent language.
- [§3.2] The reference to 'the lower panel, second from the right' is ambiguous for Fig. 10; specify the redshift bin instead.
- [Abstract and §4] The abstract and summary state the z=0 size–mass agreement without repeating the §2.4 caveat that the Rstar50 relation is a calibration target. A brief reminder would help readers weigh the evidence appropriately.
Circularity Check
Acknowledged built-in z=0 size match; independent z>0 and angular-momentum tests carry the central claim
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self definitional
[Section 2.4, 'Calibration versus prediction in simulation-observation comparisons'; also Abstract and Fig. 4 caption]
"COLIBRE subgrid models for supernova and AGN feedback were calibrated to reproduce the median z=0 galaxy size–mass relation of Hardwick et al. (2022)... comparisons of simulated and observed size–mass relations based on similar size definitions are not fully independent tests of the model, and some level of agreement is expected by construction if the calibration was successful."
The z=0 two-dimensional stellar half-mass radius relation Rstar50-Mstar is the calibration target itself: subgrid parameters were tuned to match Hardwick et al. (2022). Reporting that z=0 Rstar50 agrees with observed size-mass relations therefore relies in part on a quantity that was fitted, not predicted. The paper explicitly flags this ('not fully independent... expected by construction'), so the circularity is limited to that comparison; the z>0 evolution, alternative size definitions, morphology splits, and all angular-momentum relations are not calibration targets and remain independent.
full rationale
The only concrete reduction from inputs to outputs is the z=0 stellar half-mass radius comparison against the Hardwick et al. (2022) SMR, which Section 2.4 admits was used to calibrate the subgrid physics. That admission is a stated limitation and should be weighed: it makes the generic Abstract claim 'COLIBRE reproduces observed size-mass relations at z=0' partially built-in. However, the paper does not present the Hardwick Rstar50 comparison as a prediction; it labels it as calibration and explicitly lists the genuinely predictive tests: z>0 size evolution (Sections 3.2), the stellar and baryonic angular momentum-mass relations (Sections 3.3-3.4), R1, light-weighted radii in multiple bands, baryonic sizes, and morphology/SFR splits. Those tests are not determined by the calibration and provide independent content. The 'preliminary dust tests' invoked to explain the z>2 size discrepancy are unpublished and not forward-modelled, but this is an evidentiary gap / correctness risk, not circular reasoning: the paper does not use the dust adjustment as a derived prediction of the model. No load-bearing self-citation chain or imported uniqueness theorem appears; COLIBRE is validated against external, machine-independent observational datasets. Overall the central claim does not reduce to its inputs, so the score is moderate rather than high.
Assumptions & free parameters
free parameters (6)
- Subgrid feedback calibration parameters =
not quoted in this paper
- AGN coupling efficiency =
not quoted
- Resolution-dependent manual subgrid adjustments =
not quoted
- Star-forming main-sequence fit (eq. 1) =
log[sSFR/Gyr^-1] = 0.40z - 0.04z^2 - 0.73
- Observed stellar-mass scatter sigma_logM =
min(0.1 + 0.1z, 0.3) dex
- Morphology and gas-fraction thresholds =
D/T >= 0.6 / <= 0.3; fgas >= 0.6 / <= 0.3; nS <= 1.5 / >= 2.5
assumptions (5)
- domain assumption Lambda-CDM cosmology with DES Y3 '3x2pt + All Ext.' parameters and second-order LPT initial conditions
- domain assumption SPHENIX SPH and the subgrid models for cooling, star formation, feedback, and dust formation accurately capture the physics determining galaxy sizes and angular momentum
- domain assumption Stellar population synthesis models (Bruzual & Charlot 2003) and the dust-free rest-frame GAMA-band magnitudes correctly represent observed light distributions
- domain assumption Observational datasets (van der Wel 2024, Song 2025, SPARC, GAMA, xGASS) have systematic uncertainties small enough for the claimed agreement and discrepancies to be meaningful
- domain assumption Convergence: galaxies with M* above 100–1000 times the baryonic particle mass are numerically resolved so that resolution does not change the relations
Cite this review
Pith. "Pith review of The evolution of the sizes and angular momentum content of galaxies in the COLIBRE simulations." pith.science (2026). https://pith.science/paper/DS5HLVU7
@misc{pith2026260326200,
author = {Pith},
title = {Pith review of: The evolution of the sizes and angular momentum content of galaxies in the COLIBRE simulations},
year = {2026},
howpublished = {\url{https://pith.science/paper/DS5HLVU7}},
note = {Machine review of arXiv:2603.26200}
}
abstract
We analyse the sizes and specific angular momentum content of galaxies in the Colibre cosmological hydrodynamical simulations spanning two orders of magnitude in mass resolution. We compare the predicted size-mass and angular momentum-mass relations to a broad range of observational measurements spanning redshifts $z=0$ to $4$. At $z=0$, Colibre reproduces observed size-mass relations over the sampled mass range $10^8 \lesssim M_\star/{\rm M_\odot}\lesssim 10^{11.5}$, and for multiple size definitions, including two- and three-dimensional stellar half-mass radii, half-light radii across several wavelengths, as well as alternative measures such as baryonic half-mass radii and characteristic radii defined by stellar surface density thresholds. The simulations also recover the observed segregation of galaxies in the size-mass plane by morphological type and star formation rate, and reproduce the distinct, approximately parallel sequences followed by star-forming discs and quenched spheroids in the stellar specific angular momentum-mass plane. The angular momentum content of star-forming Colibre galaxies matches that of observed systems out to $z\approx 1.5$. At higher redshifts, massive galaxies ($ 10^{9.5}\lesssim M_\star/{\rm M_\odot}\lesssim 10^{11}$) in the simulations are somewhat smaller than observed, and the separation between star-forming and passive populations in the size-mass plane is reduced relative to observations, while at lower masses the agreement remains good. This apparent discrepancy may reflect the effects of dust attenuation, which is neglected in our analysis and may preferentially obscure the central regions of observed systems. Overall, our findings highlight the close connection between galaxy size, angular momentum, and morphology over cosmic time.
Figures
Figures from the paper (11 more)
Forward citations
Cited by 3 Pith papers
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The influence of feedback on the baryonic content of haloes in the COLIBRE simulations
The halo gas fraction–halo mass relation in COLIBRE is non-monotonic and depends strongly on the subgrid AGN feedback model, with jet-hybrid feedback producing lower group gas fractions that better match eROSITA and k...
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The morphologies of present-day galaxies in the COLIBRE simulations
In COLIBRE, today's galaxy morphology is set mostly by stellar mass: galaxies near M*≈1–2×10^10 M⊙ are the most disc-dominated, above ~2×10^11 M⊙ everything is spheroidal, and halo properties matter only weakly at fixed mass.
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How large can galaxies be? Ultra-deep imaging of IC 1101, the most extended known galaxy
Ultra-deep imaging places the edge of IC 1101's main stellar body at ~260 kpc, a ~520 kpc diameter that is the largest galaxy edge measured to date.
Reference graph
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Reviewed August 2, 2026 · model on record in the stance chip above.
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