{"id":"94ac199a-c18e-4914-ac9a-23a5a771f394","arxiv_id":"2603.26200","paper_version":2,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"The COLIBRE simulations reproduce observed galaxy size–mass and angular-momentum–mass relations over z=0–4, with mild high-redshift discrepancies.","lead":"COLIBRE, a suite of high-resolution cosmological simulations, produces galaxies whose sizes and angular momentum match observations from today to redshifts where the Universe was only about 1.5 billion years old. This matters because simulations that get these basic properties right can be trusted to study how galaxies grow and spin up over cosmic time.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"High-z size discrepancy rests on an unpublished dust test; full mock-observation comparison is needed before claiming success at z>2.","rationale":"The reader's weakest assumption was exactly this: neglecting dust attenuation leaves the u-band and z>1 comparisons uncleaned, with no forward-modelled test. I agree, and I sharpen the concern: the paper's own 'preliminary tests' are the only bridge between the observed z>2 size excess and the simulated deficit, yet they are neither shown nor validated against the observational measurement pipeline. This is load-bearing because the central claim explicitly extends to z=4 and the abstract highlights the apparent discrepancy as possibly dust-related. However, the paper is transparent about the limitation, flags it as future work, and does not overstate the high-z agreement—it says 'somewhat smaller than observed' and 'may reflect' dust. Therefore the reader's CONDITIONAL verdict is appropriate; no verdict change is needed. Independent support includes the convergence tests (Appendix A), aperture tests (Appendix C), and the fact that the z=0 multiple-size-definition agreement and j-M relations are predictive, not calibrated. The concrete test above would settle whether the high-z discrepancy is resolved by dust or remains a physical shortcoming, which is the key uncertainty in the strongest claim.","tokens_in":35974,"tokens_out":4046,"duration_ms":46300,"concrete_test":"Reproduce the z=2–4 size comparison using full mock observations: generate rest-frame ~1 µm images of L200m6 and L400m7 galaxies with a 3D dust radiative transfer code (e.g. SKIRT) using COLIBRE dust masses and stellar SEDs; convolve with JWST/HST PSFs at observed wavelengths; add realistic noise; measure half-light radii with GALFIT; apply the same resolution and quality cuts as Song et al. (2025). Compare the resulting mock R_1µm,50–M* relation to Song et al. (2025) in each redshift bin. If the mock sizes increase by the required ~0.1–0.5 dex and match the observed medians, the dust explanation is confirmed; if not, the high-z size deficit is a genuine physical discrepancy that the central claim must acknowledge.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central claim of an observationally successful framework across cosmic time depends on the high-redshift size–mass comparison. At z>2, COLIBRE galaxies with 10^9.5 ≲ M*/M⊙ ≲ 10^11 are ~0.1–0.5 dex smaller than observed (Fig. 10). The only quantitative reconciliation offered is a single sentence in Section 3.2: 'preliminary tests indicate that accounting for dust attenuation increases the apparent sizes of intermediate-mass COLIBRE galaxies' by ~0.1–0.4 dex at z=3. This test is not shown, not described, and is not a full forward model: it omits PSF convolution, the GALFIT measurement procedure, and the resolution/quality cuts used by Song et al. (2025). Without this test, the z>2 discrepancy is an unresolved tension in the strongest claim, not a confirmed success. The same issue affects the z=0 u-band R_u,90 comparison (Fig. 6), where the simulated values are slightly low and dust is again invoked ad hoc. If the preliminary dust result is an artifact of a simplified model or of measuring intrinsic rather than observed light profiles, the simulation genuinely under-predicts high-redshift galaxy sizes, undermining the headline claim of agreement at z=0–4. Because the paper's own caveat acknowledges this gap, the argument is not internally inconsistent, but the load-bearing premise—that dust explains the discrepancy—is untested.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","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.","tokens_in":36280,"tokens_out":9383,"duration_ms":96330,"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":[{"comment":"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.","section":"§3.2, Fig. 10; also Fig. 6"},{"comment":"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.","section":"§3.2, Fig. 9; §4 item (v)"}],"minor_comments":[{"comment":"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.","section":"Fig. 13"},{"comment":"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.","section":"Throughout"},{"comment":"The caption says sizes are for 'random orientations', whereas §2.3 defines 2D sizes as averages over three orthogonal projections. Use consistent language.","section":"Fig. 10 caption"},{"comment":"The reference to 'the lower panel, second from the right' is ambiguous for Fig. 10; specify the redshift bin instead.","section":"§3.2"},{"comment":"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.","section":"Abstract and §4"}],"recommendation":"major_revision","confidential_remarks":"This is a strong paper that is being held back by its own high-z caveat. The reader's stress-test concern is legitimate: the single unpublished dust sentence cannot carry the z>2 conclusion. I would accept a revision that either provides the mock-observation test or appropriately limits the headline claim; I do not see a basis for rejection, since the z<2 and angular-momentum results are valuable and appear supported by the presented data."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is the first systematic size–mass and j–M comparison for COLIBRE, and it is a careful piece of work. The paper does something right that many simulation papers still get wrong: Section 2.4 explicitly separates what is calibrated from what is genuinely predictive. The z=0 half-mass radius match to Hardwick et al. is acknowledged as partly built in; the interesting claims rest on alternative size definitions (R1, baryonic radii, multi-band half-light), the morphology and star-forming/passive splits, and the j–M relations, none of which were used in calibration. The convergence testing across two orders of magnitude in resolution looks genuinely good, and the appendices on aperture effects, satellites, and mass scatter are exactly the checks you want to see. The j–M agreement at z=0 and out to z≈1.5, including the morphological separation, is a solid independent result.\n\nThe soft spots are in proportion. The high-z discrepancy (z>2, M* = 10^9.5–10^11) is real: the simulations are 0.1–0.5 dex smaller than observed, and the paper says preliminary tests suggest dust attenuation increases simulated sizes by 0.1–0.4 dex at z=3. But that test is not shown, not described, and does not include PSF convolution or the GALFIT measurement procedure used by Song et al. It is an unexplained single sentence. That does not sink the paper — the z=0 conclusions and the z≤1.5 j–M results stand independently — but it does mean the headline \"successful across cosmic time\" is carrying weight from an untested premise. The slightly low u-band R_u,90 at z=0 is the same issue in miniature, and the paper handles it well by saying dust may be responsible rather than overclaiming.\n\nData availability is a minor annoyance: the COLIBRE-specific modules are not public yet, and the data are \"available upon request,\" which is weaker than it could be. That is fixable, not fatal.\n\nIf I were the editor, I would send this to referees. The transparency and the range of independent tests make it worth the referees' time. But I would ask the authors to either show the dust test in detail or explicitly soften the high-z part of the abstract and summary. As written, the high-redshift claim overreaches slightly, and the referee report should say so.","headline":"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.","tokens_in":722,"tokens_out":887,"would_cite":true,"duration_ms":23232,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"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.","keywords":["galaxy size–mass relation","specific angular momentum","cosmological hydrodynamical simulations","galaxy evolution","galaxy morphology","star-forming and passive galaxies","galaxy scaling relations"],"falsifier":"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.","tokens_in":35815,"feed_emoji":"🌀","tokens_out":11798,"duration_ms":116218,"temperature":0.7,"pith_summary":"The paper aims to establish that a single cosmological hydrodynamical simulation package, COLIBRE, produces realistic galaxy sizes and angular momenta across most of cosmic time. At z=0 the simulations match observed size–mass relations over stellar masses from roughly 10^8 to 10^11.5 solar masses, for half-mass and half-light radii in several bands, for a surface-density-threshold radius, and for baryonic half-mass radii. They also reproduce the observed parallel sequences of star-forming discs and quenched spheroids in the specific-angular-momentum–mass plane, and the angular momentum of star-forming galaxies matches observations out to z≈1.5. At z≳2 massive simulated galaxies are smaller than observed, which the paper suggests may be caused by dust attenuation that is absent from the simulated size measurements. This matters because it ties size, angular momentum, and morphology to a common set of physical processes and offers a tested setting for future studies of how galaxies acquire and lose angular momentum.","feed_headline":"Simulated galaxies match observed sizes and spin out to z=1.5","feed_subtitle":"Simulations now match not just galaxy masses but how big and how spinny galaxies are at each mass and age.","key_machinery":"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","core_discovery":"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","pith_inferences":["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."],"forward_implications":["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."],"fun_headline_variants":["Simulations reproduce galaxy sizes and angular momentum","COLIBRE sims match galaxy size and spin observations","Galaxy size and spin matched by new simulations","Simulations now fit observed galaxy size and spin","COLIBRE matches galaxy size and spin across redshift"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"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.","fun_headline_variants_meta":{"raw":{"variants":["Simulations reproduce galaxy sizes and angular momentum","COLIBRE sims match galaxy size and spin observations","Galaxy size and spin matched by new simulations","Simulations now fit observed galaxy size and spin","COLIBRE matches galaxy size and spin across redshift"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000573,"raw_usage":{"total_tokens":2600,"prompt_tokens":859,"completion_tokens":1741,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":603,"completion_tokens_details":{"reasoning_tokens":1682}},"tokens_in":603,"tokens_out":1741,"duration_ms":14026,"temperature":1.0,"reasoning_tokens":1682,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T17:16:41.325576+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"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.","supporting_citations":[],"review_version":1}