REVIEW 2 major objections 4 minor 48 references
A diffusion model can generate galaxy catalogues with correct clustering statistics directly from a dark matter density field, without ever identifying haloes.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-01 11:37 UTC pith:DBJXK4WX
load-bearing objection A practical, honest step forward in diffusion-based mock generation; the independent-patch assumption is the load-bearing caveat and higher-order statistics are untested. the 2 major comments →
From Dark Matter to Galaxies: Halo-Free Mock Generation via Conditional Point-Cloud Diffusion
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper's central claim is that a point-cloud diffusion model conditioned on a low-resolution dark matter density field can act as a direct, halo-free mapping from matter to galaxies. The model inputs a 3D density grid with ~1.2 Mpc voxels, downsampled to mimic a very low-resolution dark-matter-only simulation, and outputs galaxy points with positions, star formation rate, and stellar mass. Trained on a hydrodynamical simulation, the generated catalogs match the reference simulation's galaxy properties, clustering amplitude, and cross-correlation with the dark matter field over a wide range of scales. The authors further show that the model generates galaxies in halos below the nominal res
What carries the argument
The central object is a conditional point-cloud diffusion model built on the Elucidated Diffusion Model (EDM) framework: a transformer-based denoising network that takes noisy galaxy point sets and a voxelized dark matter density cube as input, and learns to denoise the points into realistic galaxy configurations with physical attributes. Self-attention among galaxy tokens captures galaxy–galaxy correlations, while cross-attention to the density cube injects the dark matter environment. The independent-patch generation with tiling is the mechanism that makes large volumes tractable, and the stochastic sampling step is what enables the model to marginalize over unresolved small-scale structur
Load-bearing premise
The method assumes that galaxy formation is governed primarily by the local dark matter density within patches of about 19 Mpc, so that galaxies in one patch are statistically independent of everything outside it; if long-range environmental effects or correlations beyond a few megaparsecs matter, the generated mocks will miss them.
What would settle it
Measure the galaxy bispectrum (or three-point correlation function) of the generated catalogue against the reference simulation over the full test volume; a significant deviation on scales near or above the patch size would falsify the independent-patch assumption and reveal missing large-scale mode coupling.
If this is right
- If the approach holds up, mock galaxy catalogues for redshift surveys and line-intensity mapping can be generated directly from cheap, low-resolution dark-matter-only simulations, removing the need for halo finders and subhalo abundance matching.
- The ability to generate galaxy populations below the resolution limit means that faint or low-mass galaxies, which dominate future intensity-mapping signals, can be modelled from density fields that do not resolve their host halos.
- Because each patch is generated independently, the method is naturally parallelizable and can scale to arbitrarily large survey volumes by tiling.
- The point-cloud output is a continuous representation, allowing the catalogues to be trimmed or reshaped to survey geometries and selection functions without a fixed grid.
- Extending the per-galaxy feature vector (e.g., adding velocities, gas masses, metallicities) is computationally cheap in this framework, making richer mock observables feasible at little extra cost.
Where Pith is reading between the lines
- The paper's patch-independence assumption (that galaxy formation is local and correlations die out within about 20 Mpc) is only validated for two-point statistics; if environmental effects like assembly bias or tidal alignment act on larger scales, they would show up first in the bispectrum or marked correlation functions, so those should be tested before using the mocks for precision cosmology.
- The observed ability to marginalize over unresolved structure suggests the model could be trained on even lower-resolution inputs, such as fast particle-mesh simulations, and still produce believable small-scale galaxy populations — a testable extension that would further lower the cost of mock production.
- Since the model learns a conditional distribution rather than a deterministic map, the generated realizations naturally encode the stochastic scatter of galaxy formation, which could be exploited for simulation-based inference where field-level likelihoods are needed.
- The patch size is a hyperparameter; training the same model on larger conditioning volumes (say 50–100 Mpc) would directly test whether large-scale modes beyond the current patch are needed for high-order statistics, and would set a practical limit on the method's accuracy.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a conditional diffusion model (EDM) that generates galaxy point clouds — positions, star formation rate, and stellar mass — directly from a three-dimensional dark-matter density field, without any halo or subhalo identification. The model is trained on IllustrisTNG300 at z=2, using a heavily downsampled dark-matter density field as input (1.2 Mpc voxels, 16^3 voxels per 18.9 Mpc patch) and TNG300-1 galaxies with SFR > 1 M_sun/yr as targets. Generation is performed patch-by-patch and the patches are tiled to cover a held-out (151.3 Mpc)^3 test volume. The paper reports that the generated catalogues reproduce the one- and two-dimensional distributions of SFR and stellar mass, the galaxy auto-power spectrum, and the galaxy–DM cross-power spectrum, and that the model statistically populates structures below the nominal resolution limit of the input density field. An optimized sampling schedule yields a ~10 second generation time for the full test volume. An autoregressive patch-generation variant and sampling-schedule tuning are presented in appendices.
Significance. If the claims hold, this is a useful proof-of-concept for fast, halo-free mock generation that could be relevant for survey ensembles and simulation-based inference. The clean train/test split, the use of multiple realizations for property distributions, the explicit comparison to a halo-mass-threshold baseline, and the careful tuning of the sampling schedule are strengths. The paper is appropriately cautious in stating that two-point agreement does not guarantee higher-order fidelity. However, the quantitative support for the central clustering claim is weakened by the absence of error bars on the power-spectrum ratios, and the stated survey-mock motivation goes beyond what is actually validated.
major comments (2)
- [§3.2, Fig. 4] The auto- and cross-power spectra are displayed as single ratio curves with no uncertainty. The abstract and Section 3.2 claim that the generated catalogues 'accurately reproduce' these spectra, but with a single realization the reader cannot assess the scatter, especially on large scales where sample variance is significant. Since 100 independent realizations are already used for Fig. 3, the authors should show the mean and standard deviation (or an error band) of P_gen and P_X,gen over these realizations, and state how many realizations are used for the power-spectrum estimate. This is needed to support the central clustering claim.
- [§2.1, §5, Abstract] Patch-wise independent generation is validated only for two-point statistics; the paper explicitly acknowledges in §3.2 that higher-order correlations and mode-mode coupling are not guaranteed. However, the abstract and Introduction motivate the method as a 'practical engine for producing large mock ensembles' for surveys, which rely on accurate covariance matrices and non-Gaussian statistics. This is a mismatch between the validated scope and the stated applications. Either add a higher-order test (e.g., reduced bispectrum or another non-Gaussian statistic) or explicitly scope the claims to two-point statistics and temper the abstract/motivation accordingly.
minor comments (4)
- [§2.1] 'containing ∼2903 DM particles' appears to be a typo; presumably 290^3 or another power-of-ten count is intended. Please correct the missing superscript.
- [§3.2] The sentence 'The black dashed line show that of TNG galaxies...' should be 'shows'; also note that the dashed curve uses a 10^12 M_sun/h halo-mass threshold, which corresponds to ~20 particles in the downsampled input, while §2.1 states that reliable haloes require ~100 particles (M_h ≳ 4.5×10^12). Clarifying this would prevent over-interpretation of the comparison.
- [Fig. 4 and Fig. A2] The x-axis label 'k [Mpc 1]' should be 'k [Mpc^{-1}]'.
- [Eq. (1)] The normalization expression is ambiguous: '2 log s − a b − a − 1' should be written with clear parentheses, e.g., 2(log s − a)/(b − a) − 1.
Circularity Check
No significant circularity: the method is trained on TNG patches and validated against a held-out TNG volume with independently selected hyperparameters.
full rationale
The paper's central derivation is self-contained and not circular. The conditional diffusion model is trained on patches drawn from IllustrisTNG and evaluated on a reserved (151.3 Mpc)^3 test volume that was not used in training or validation; the reported SFR/stellar-mass distributions, auto-power spectrum, and cross-power spectrum are measured from generated samples and compared directly to TNG data. No parameter is fitted to these test statistics: the sampling schedule parameter alpha=0.6 is chosen on validation data via the Wasserstein distance in Fig. B2, and the number of sampling steps is also a validation decision, not a fit to the final power spectra. The comparison against the resolved-halo baseline (black dashed line in Fig. 3) is a diagnostic, not an input. The self-citations to Moriwaki et al. (2026) and Osato & Okumura (2023) are used only as context or comparison (related work and a general statement about hydrodynamical simulation mocks) and do not supply any load-bearing premise for the results shown. The paper's own caveat that two-point agreement does not guarantee higher-order correlations (Section 3.2) is an acknowledged limitation rather than a circular step. Thus there is no identifiable reduction of any prediction to its own inputs, and no circularity score above 0 is warranted.
Axiom & Free-Parameter Ledger
free parameters (5)
- SFR normalization bounds (a,b) =
(0,3)
- Stellar mass normalization bounds (a,b) =
(7,12)
- DM density normalization factor =
log10(rho)/5
- Sampling schedule parameter alpha =
0.6
- Patch size =
18.9 Mpc
axioms (4)
- domain assumption The TNG simulation is a reliable representation of galaxy formation.
- domain assumption The downsampled TNG300-3-Dark density field is a valid proxy for a low-resolution DM-only simulation.
- domain assumption Galaxy formation is primarily governed by local physical conditions within a patch of ~19 Mpc.
- domain assumption Stochasticity in star formation justifies comparing statistical distributions rather than individual objects.
read the original abstract
We present a diffusion-based generative model for constructing realistic galaxy catalogues from dark matter density fields. The model takes a three-dimensional dark matter density field as input and generates galaxies directly as a point cloud with positions and physical properties, including star formation rate (SFR), without identifying dark matter haloes or subhaloes. We train the model on galaxy catalogues from the IllustrisTNG hydrodynamical simulation. The generated catalogues reproduce the spatial correspondence between galaxies and the underlying dark matter field, preferentially populating dense regions and filamentary structures. They also accurately reproduce the one- and two-dimensional distributions of SFR and stellar mass, the galaxy auto-power spectrum, and the galaxy--dark matter cross-power spectrum. The model generates galaxies associated with structures below the nominal resolution of the input density field by marginalising over unresolved small-scale structure rather than relying on a resolved halo catalogue. With an optimised diffusion sampling schedule, it generates a catalogue with SFR $> 1 ~\rm M_\odot/yr$ over a $(151.3 ~\rm Mpc)^3$ volume in approximately 10 seconds on a single GPU. Our model therefore provides a practical engine for producing large mock ensembles for upcoming galaxy redshift surveys and line-intensity mapping experiments, and offers a path toward simulation-based inference that bypasses halo finding and directly connects field-level dark matter statistics to observable galaxy populations.
Figures
Reference graph
Works this paper leans on
-
[1]
Aguirre J., STARFIRE Collaboration 2018, in American Astronomical Society Meeting Abstracts \#231. p. 328.04
2018
-
[2]
Beltagy I., Peters M. E., Cohan A., 2020, @doi [arXiv e-prints] 10.48550/arXiv.2004.05150 , https://ui.adsabs.harvard.edu/abs/2020arXiv200405150B p. arXiv:2004.05150
-
[3]
CCAT-Prime Collaboration et al., 2023, @doi [ ] 10.3847/1538-4365/ac9838 , https://ui.adsabs.harvard.edu/abs/2023ApJS..264....7C 264, 7
-
[4]
CONCERTO Collaboration et al., 2020, @doi [ ] 10.1051/0004-6361/202038456 , https://ui.adsabs.harvard.edu/abs/2020A&A...642A..60C 642, A60
-
[5]
T., et al., 2014, in Holland W
Crites A. T., et al., 2014, in Holland W. S., Zmuidzinas J., eds, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series Vol. 9153, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII. p. 91531W, @doi 10.1117/12.2057207
-
[6]
Cuesta-Lazaro C., Mishra-Sharma S., 2024, @doi [ ] 10.1103/PhysRevD.109.123531 , https://ui.adsabs.harvard.edu/abs/2024PhRvD.109l3531C 109, 123531
-
[7]
DESI Collaboration et al., 2016, @doi [arXiv e-prints] 10.48550/arXiv.1611.00036 , https://ui.adsabs.harvard.edu/abs/2016arXiv161100036D p. arXiv:1611.00036
-
[8]
Dosovitskiy A., et al., 2020, @doi [arXiv e-prints] 10.48550/arXiv.2010.11929 , https://ui.adsabs.harvard.edu/abs/2020arXiv201011929D p. arXiv:2010.11929
-
[9]
Euclid Collaboration et al., 2022, @doi [ ] 10.1051/0004-6361/202141938 , https://ui.adsabs.harvard.edu/abs/2022A&A...662A.112E 662, A112
-
[10]
Euclid Collaboration et al., 2025, @doi [ ] 10.1051/0004-6361/202450810 , https://ui.adsabs.harvard.edu/abs/2025A&A...697A...1E 697, A1
-
[11]
Ho J., Jain A., Abbeel P., 2020, @doi [arXiv e-prints] 10.48550/arXiv.2006.11239 , https://ui.adsabs.harvard.edu/abs/2020arXiv200611239H p. arXiv:2006.11239
-
[12]
K., Cranmer M., Melchior P., Ho S., Somerville R
Jespersen C. K., Cranmer M., Melchior P., Ho S., Somerville R. S., Gabrielpillai A., 2022, @doi [ ] 10.3847/1538-4357/ac9b18 , https://ui.adsabs.harvard.edu/abs/2022ApJ...941....7J 941, 7
-
[13]
Jo Y., Kim J.-h., 2019, @doi [ ] 10.1093/mnras/stz2304 , https://ui.adsabs.harvard.edu/abs/2019MNRAS.489.3565J 489, 3565
-
[14]
Kamdar H. M., Turk M. J., Brunner R. J., 2016, @doi [ ] 10.1093/mnras/stv2981 , https://ui.adsabs.harvard.edu/abs/2016MNRAS.457.1162K 457, 1162
-
[15]
Karras T., Aittala M., Aila T., Laine S., 2022, @doi [arXiv e-prints] 10.48550/arXiv.2206.00364 , https://ui.adsabs.harvard.edu/abs/2022arXiv220600364K p. arXiv:2206.00364
-
[16]
Kingma D. P., Ba J., 2014, @doi [arXiv e-prints] 10.48550/arXiv.1412.6980 , https://ui.adsabs.harvard.edu/abs/2014arXiv1412.6980K p. arXiv:1412.6980
-
[17]
LSST Science Collaboration et al., 2009, @doi [arXiv e-prints] 10.48550/arXiv.0912.0201 , https://ui.adsabs.harvard.edu/abs/2009arXiv0912.0201L p. arXiv:0912.0201
-
[18]
Li Y., et al., 2022, @doi [arXiv e-prints] 10.48550/arXiv.2211.09958 , https://ui.adsabs.harvard.edu/abs/2022arXiv221109958L p. arXiv:2211.09958
-
[19]
Liu Z., Lin Y., Cao Y., Hu H., Wei Y., Zhang Z., Lin S., Guo B., 2021, @doi [arXiv e-prints] 10.48550/arXiv.2103.14030 , https://ui.adsabs.harvard.edu/abs/2021arXiv210314030L p. arXiv:2103.14030
-
[20]
C., et al., 2023, in Machine Learning for Astrophysics
Lovell C. C., et al., 2023, in Machine Learning for Astrophysics. p. 21 ( @eprint arXiv 2307.06967 ), @doi 10.48550/arXiv.2307.06967
-
[21]
Marinacci F., et al., 2018, @doi [ ] 10.1093/mnras/sty2206 , https://ui.adsabs.harvard.edu/abs/2018MNRAS.480.5113M 480, 5113
-
[22]
Mishra S., Trotta R., Viel M., 2026a, @doi [arXiv e-prints] 10.48550/arXiv.2601.14377 , https://ui.adsabs.harvard.edu/abs/2026arXiv260114377M p. arXiv:2601.14377
-
[23]
Mishra S., Trotta R., Viel M., 2026b, @doi [ ] 10.1093/mnras/staf2071 , https://ui.adsabs.harvard.edu/abs/2026MNRAS.545f2071M 545, staf2071
-
[24]
Moriwaki K., Jun R. L., Osato K., Yoshida N., 2026, @doi [ ] 10.1093/mnras/staf2124 , https://ui.adsabs.harvard.edu/abs/2026MNRAS.545f2124M 545, staf2124
-
[25]
Naiman J. P., et al., 2018, @doi [ ] 10.1093/mnras/sty618 , https://ui.adsabs.harvard.edu/abs/2018MNRAS.477.1206N 477, 1206
-
[26]
Nelson D., et al., 2018, @doi [ ] 10.1093/mnras/stx3040 , https://ui.adsabs.harvard.edu/abs/2018MNRAS.475..624N 475, 624
-
[27]
Nelson D., et al., 2019, @doi [Computational Astrophysics and Cosmology] 10.1186/s40668-019-0028-x , https://ui.adsabs.harvard.edu/abs/2019ComAC...6....2N 6, 2
-
[28]
Nguyen T., et al., 2024, @doi [arXiv e-prints] 10.48550/arXiv.2409.02980 , https://ui.adsabs.harvard.edu/abs/2024arXiv240902980N p. arXiv:2409.02980
-
[29]
Osato K., Okumura T., 2023, @doi [ ] 10.1093/mnras/stac3582 , https://ui.adsabs.harvard.edu/abs/2023MNRAS.519.1771O 519, 1771
-
[30]
Oxholm T., et al., 2020, in American Astronomical Society Meeting Abstracts \#236. p. 244.02
2020
-
[31]
Pandey S., Lovell C. C., Modi C., Wandelt B. D., 2025, @doi [arXiv e-prints] 10.48550/arXiv.2511.08438 , https://ui.adsabs.harvard.edu/abs/2025arXiv251108438P p. arXiv:2511.08438
-
[32]
Paszke A., et al., 2019, @doi [arXiv e-prints] 10.48550/arXiv.1912.01703 , https://ui.adsabs.harvard.edu/abs/2019arXiv191201703P p. arXiv:1912.01703
-
[33]
Peebles W., Xie S., 2022, @doi [arXiv e-prints] 10.48550/arXiv.2212.09748 , https://ui.adsabs.harvard.edu/abs/2022arXiv221209748P p. arXiv:2212.09748
-
[34]
Pillepich A., et al., 2018a, @doi [ ] 10.1093/mnras/stx2656 , https://ui.adsabs.harvard.edu/abs/2018MNRAS.473.4077P 473, 4077
-
[35]
Pillepich A., et al., 2018b, @doi [ ] 10.1093/mnras/stx3112 , https://ui.adsabs.harvard.edu/abs/2018MNRAS.475..648P 475, 648
-
[36]
Rombach R., Blattmann A., Lorenz D., Esser P., Ommer B., 2021, @doi [arXiv e-prints] 10.48550/arXiv.2112.10752 , https://ui.adsabs.harvard.edu/abs/2021arXiv211210752R p. arXiv:2112.10752
-
[37]
Rouhiainen A., M \"u nchmeyer M., Shiu G., Gira M., Lee K., 2024, @doi [ ] 10.1103/PhysRevD.109.123536 , https://ui.adsabs.harvard.edu/abs/2024PhRvD.109l3536R 109, 123536
-
[38]
Probabilistic Galaxy Field Generation with Diffusion Models
Sether T., Giusarma E., Reyes-Hurtado M., 2024, @doi [arXiv e-prints] 10.48550/arXiv.2412.05131 , https://ui.adsabs.harvard.edu/abs/2024arXiv241205131S p. arXiv:2412.05131
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.2412.05131 2024
-
[39]
Shirasaki M., Ikeda S., 2024, @doi [The Open Journal of Astrophysics] 10.33232/001c.118104 , https://ui.adsabs.harvard.edu/abs/2024OJAp....7E..42S 7, 42
-
[40]
Song Y., Sohl-Dickstein J., Kingma D. P., Kumar A., Ermon S., Poole B., 2020, @doi [arXiv e-prints] 10.48550/arXiv.2011.13456 , https://ui.adsabs.harvard.edu/abs/2020arXiv201113456S p. arXiv:2011.13456
-
[41]
Spergel D., et al., 2015, @doi [arXiv e-prints] 10.48550/arXiv.1503.03757 , https://ui.adsabs.harvard.edu/abs/2015arXiv150303757S p. arXiv:1503.03757
-
[42]
Springel V., White S. D. M., Tormen G., Kauffmann G., 2001, @doi [ ] 10.1046/j.1365-8711.2001.04912.x , https://ui.adsabs.harvard.edu/abs/2001MNRAS.328..726S 328, 726
arXiv 2001
-
[43]
Springel V., et al., 2018, @doi [ ] 10.1093/mnras/stx3304 , https://ui.adsabs.harvard.edu/abs/2018MNRAS.475..676S 475, 676
-
[44]
Takada M., et al., 2014, @doi [ ] 10.1093/pasj/pst019 , https://ui.adsabs.harvard.edu/abs/2014PASJ...66R...1T 66, R1
-
[45]
Vaswani A., Shazeer N., Parmar N., Uszkoreit J., Jones L., Gomez A. N., Kaiser L., Polosukhin I., 2017, @doi [arXiv e-prints] 10.48550/arXiv.1706.03762 , https://ui.adsabs.harvard.edu/abs/2017arXiv170603762V p. arXiv:1706.03762
-
[46]
Diffusion Models in 3D Vision: A Survey
Wang Z., Li D., Wu Y., He T., Bian J., Jiang R., 2024, @doi [arXiv e-prints] 10.48550/arXiv.2410.04738 , https://ui.adsabs.harvard.edu/abs/2024arXiv241004738W p. arXiv:2410.04738
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.2410.04738 2024
-
[47]
Zaheer M., et al., 2020, @doi [arXiv e-prints] 10.48550/arXiv.2007.14062 , https://ui.adsabs.harvard.edu/abs/2020arXiv200714062Z p. arXiv:2007.14062
-
[48]
From Dark Matter to Galaxies with Convolutional Networks
Zhang X., Wang Y., Zhang W., Sun Y., He S., Contardo G., Villaescusa-Navarro F., Ho S., 2019, @doi [arXiv e-prints] 10.48550/arXiv.1902.05965 , https://ui.adsabs.harvard.edu/abs/2019arXiv190205965Z p. arXiv:1902.05965
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.1902.05965 2019
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.