REVIEW 4 major objections 4 minor 1 cited by
The SRG/eROSITA all-sky survey: The morphologies of clusters of galaxies I: A catalogue of morphological parameters
T0 review · 4 major / 4 minor · reviewed 2026-08-09 · deepseek-v4-flash
Pith's one-line read The first SRG/eROSITA all-sky morphological catalogue, covering more than twelve thousand optically confirmed galaxy clusters, argues that X-ray selection strongly favors concentrated, likely relaxed clusters, with log concentrations…
desk verdict Solid eRASS1 morphology catalogue with genuinely new forward-modelled parameters; the headline 0.3 dex concentration offset is directionally right but quantitatively depends on AGN-free simulations that the authors honestly flag. 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 load-bearing tool is MBProj2D forward modelling of the X-ray surface brightness: the cluster image is compared with a model that includes the eROSITA PSF, background, point sources and neighbouring clusters, using a density profile with a weak inner-slope prior. Concentration is measured from the model, not the image, as $\log$ of the ratio of integrated surface brightness in apertures $0.1\,R_{500}$ and $R_{500}$ ($c_{500}$), or 80 and 800 kpc ($c_{80-800}$). Two new parameters are introduced: slosh, which transforms the radius as $S'(r,\theta) = A(H)\,S(r\,[1 + H\cos(\theta+\theta_0)])$ with $A(H)=(1-H^2)^{3/2}$ to keep the total brightness fixed, and multipole magnitudes $M_m$, which multiply the profile by $[1 + M_m\sin(m\theta+\theta_0)]\,S(r)$. Simulated clusters placed on eROSITA sky maps and run through the eSASS detection pipeline provide the selection functions and bias curves, and a two-component Gaussian mixture model fit to the bright, high-count clusters defines the $D_{\rm shape}$ and $D_{\rm comb}$ disturbance scores.
What would settle it
Take the eRASS1 clusters that fall in the deeper eRASS:4 footprint, re-measure $c_{500}$ and $c_{80-800}$ with detected point sources subtracted, and compare the median concentration with the eRASS1 values: if the median drops by roughly 0.3 dex, the claimed selection effect is not sufficient to explain the offset and the central claim fails.
Extended reading notes
Core claim
Measuring 29 morphological parameters for the 12,075 clusters with usable data from the eRASS1 catalogue, the authors find that the population is systematically more concentrated than clusters selected by the South Pole Telescope, by Planck, or in the deeper eFEDS field: the median log concentration is about 0.3 dex higher, a factor of two in the inner-to-total surface brightness ratio. They show from simulations that concentration controls whether a cluster is detected, with flat-core low-luminosity objects missed at low redshift and very peaked cool cores lost at high redshift because they look point-like; and they demonstrate that the concentration excess survives cuts in exposure time, counts, detection likelihood, and extension likelihood. For the same clusters, concentrations and central densities agree with Chandra and XMM-Newton measurements, and a matched re-analysis of SPT clusters in deeper eRASS:4 data agrees with Chandra at the one-to-one level. Image-based parameters such as power ratios, centroid shift, Gini coefficient, and photon asymmetry are strongly biased by noise and PSF, while the forward-modelled parameters are not. Finally, a two-component Gaussian mixture model applied to the shape parameters and concentration produces a disturbance score; about one quarter of bright clusters are classified as disturbed, and the paper attributes the high relaxed fraction to the survey's sensitivity to concentrated systems.
Load-bearing premise
The quantitative bias curves, reliability thresholds, and selection functions are computed from simulations that contain no AGN, so if unresolved active galactic nuclei are common in eRASS1 clusters the claimed sizes of the concentration bias and high-redshift selection effect could be wrong.
Editorial extensions
If this is right
- Cosmological analyses using eRASS1 cluster counts will need to include a concentration-dependent selection function, since detection efficiency varies strongly with both redshift and luminosity at fixed concentration.
- The published catalogue gives bias and uncertainty tables as a function of redshift and count number, so users can correct or discard image-based morphological parameters rather than comparing them blindly with other surveys.
- At low redshift the sample misses low-luminosity groups and clusters with flat surface brightness profiles; at $z \gtrsim 0.4$ it misses the most concentrated cool cores, including objects like the Phoenix cluster.
- The $\sim$0.3 dex offset between eRASS1 and SPT/Planck/eFEDS concentrations indicates that X-ray-selected samples contain roughly twice the inner flux fraction of SZ-selected samples, so statements about cool-core fractions and relaxed fractions must be survey-selection aware.
- Around a quarter of bright clusters are classified as disturbed by the combined score, while shape-only and concentration-inclusive scores can disagree for individual clusters; the two scores are provided so users can separate shape disturbance from peaked-core status.
Reading between the lines
- The selection function implies that eRASS1-based scaling relations, for example luminosity or $Y_X$ versus mass, will inherit a redshift- and luminosity-dependent bias because concentrated clusters are preferentially detected at the faint end; the authors do not work out this corollary here.
- If the concentration excess is truly selection rather than measurement, then the deeper eRASS:4 survey should show a lower median concentration for the same clusters, an effect that could be checked already with the matched-sample machinery in the paper.
- The missing high-redshift extreme cool cores could be recovered by wavelet-based or photon-based detection algorithms, which the authors note may find more flat-profile objects; this is a testable prediction of their selection model.
- Because unresolved AGN were absent from the simulations, adding a realistic AGN population would probably shift the quantitative bias curves at low luminosity and high redshift; the direction of the selection effect would likely remain but its amplitude could change.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper measures 29 morphological parameters for 12,075 clusters in the eRASS1 catalogue, introducing two forward-modelled PSF-aware parameters (slosh and multipole magnitudes). It uses MCMC and bootstrap uncertainty propagation, simulations without AGN to quantify parameter biases and the selection function, and matched subsamples against XMM-Newton, Chandra, and eFEDS. The central empirical claim is that eRASS1 clusters are systematically more concentrated than SPT, Planck ESZ, and eFEDS samples, with median log c500 around 0.3 dex higher, and that the concentration distribution depends on redshift and luminosity because the X-ray selection preferentially detects concentrated clusters.
Significance. If correct, this is an important reference catalogue and a clear demonstration of how X-ray selection shapes morphological distributions. The paper's strengths are the scale of the catalogue, the explicit treatment of the PSF through forward-modelled parameters, the detailed uncertainty propagation, and the external validation using matched samples, including 1:1 agreement between eRASS:4 and Chandra when the analysis procedures are matched. However, the quantitative central claim depends on simulations that contain no AGN, a limitation stated in Section 6.11, and on a residual 0.1 dex offset between eRASS1 and XMM/Chandra for the same clusters. These issues make the quantitative selection and bias curves insecure, although the qualitative direction of the selection effect is plausible.
major comments (4)
- [§4, §6.11, Figs. 12 and 18] The simulations used to set the quantitative selection functions and bias corrections contain no AGN, and Section 6.11 concedes that adding a point-source population would likely add bias, especially for low-luminosity, high-redshift objects. Since an unresolved central AGN adds flux inside the 0.1 R500 aperture and eROSITA's roughly 30 arcsec survey PSF cannot resolve faint core point sources, the measured eRASS1 c500 values themselves may be inflated. This matters because the claimed 0.3 dex population offset is the sum of roughly a 0.1 dex measurement offset seen in Section 7.1 and a roughly 0.2 dex selection effect; if unresolved AGN contribute to the measurement offset, the quantitative population comparison and the selection curves in Fig. 12 are not yet secure. Please quantify this by adding point-source populations to the simulations or by testing c500 against known central AGN indicators in the eRASS1 sample.
- [§7.1 and §7.2] The matched-sample comparisons show a median eRASS1 concentration about 0.1 dex higher than XMM-Newton or Chandra for the same clusters, while the matched eRASS:4 analysis gives a 1:1 relation. This residual offset is not explained, and Section 7.1 lists unresolved point sources as a possible cause. The conclusion that cuts on exposure, counts, detection likelihood, and extension likelihood do not remove the 0.3 dex offset is based on observed distributions that retain this offset; the Abstract's statement that the population difference is a selection effect is therefore stronger than the current evidence supports. The authors should either correct the eRASS1 values for the inferred measurement offset before comparing populations, or demonstrate explicitly that the offset is not AGN-related.
- [§8 and Table 5] The Gaussian mixture model is trained on the 175 brightest clusters and then applied to the full sample, including those same 175 objects, to produce the reported roughly 25% disturbed fractions. This training-on-test overlap, without cross-validation, makes the Dshape and Dcomb fractions and the two-component amplitudes in Table 5 difficult to interpret. A cross-validated or independent-sample test is needed before the disturbance classification is used as a catalogue product.
- [§4 and Fig. 12] The simulated detection pipeline is image-based rather than the photon-based pipeline used for the real catalogue, and the authors note it is less sensitive near the detection threshold. This could shift the high-redshift selection thresholds in Fig. 12, in particular the claimed loss of clusters with c80−800 of about −0.2 at z = 0.4, and should be quantified so that the quoted threshold values are not taken as final.
minor comments (4)
- [§1] In the Introduction, 'parameters are are affected by the signal to noise' contains a duplicated word.
- [§8] The text uses 'GGM' in several places, for example 'the GGM model does not show strong evidence', where 'GMM' is intended.
- [§7.2] The phrase 'For eFEDS1, we show distributions' appears to be a typo for 'For eFEDS'.
- [Table 1] The Type column codes (M, I, V, F, P) are not defined in the table itself; adding a footnote would substantially improve readability for catalogue users.
Circularity Check
No significant circularity: the central results are externally benchmarked, and the AGN-omission in simulations is a stated accuracy limitation rather than a circular reduction.
full rationale
The paper's derivation chain is self-contained and benchmarked against external data. The morphological measurements (concentration, central density, inner slope, shape parameters) come from MBProj2D fits to eRASS1 images and are compared, for matched clusters, with independent Chandra and XMM-Newton analyses (Section 7.1), where the matched eRASS:4/Chandra procedure gives a relation consistent with 1:1 for c500. The population offset of ~0.3 dex in concentration relative to SPT, Planck ESZ, and eFEDS is an empirical comparison of measured distributions, not a fitted restatement of the claim. The selection-function and bias simulations (Section 4; Figs. 12 and 18) are forward models run through the eSASS detection pipeline; they use the same parametrization as the data analysis, but they are used to characterize PSF, noise, and selection effects, not to define the measured concentrations. The paper explicitly concedes that the simulations omit AGN (Section 6.11), which is a completeness and accuracy limitation, not a circular reduction of the result to its inputs. The GMM disturbance scores (Section 8) are fitted to a bright subsample and applied to the same clusters, but this is descriptive clustering/classification rather than a prediction, and the paper checks robustness to the count threshold and number of components. Self-citations to B24 and Ghirardini et al. (2022) supply the cluster catalogue, R500 values, redshifts, and comparison values; these are published data products and external inputs, not the target result, so the citations are not load-bearing in a circular sense. No equation is defined in terms of the quantity it is claimed to predict, and no fitted parameter is renamed as a prediction.
Assumptions & free parameters
free parameters (6)
- Outer slope beta prior =
2/3 (truncated normal, width 1/3)
- Gaussian smoothing scale =
24 arcsec
- GMM component count =
2
- GMM training threshold =
800 counts
- PSF energy used for convolution =
1.486 keV survey-averaged
- Photon asymmetry annuli =
0.05, 0.12, 0.2, 0.3 and 1.0 R500
assumptions (6)
- domain assumption Clusters are isothermal with 0.3 solar metallicity in the fitted emissivity model.
- ad hoc to paper The simplified density profile (beta model with inner powerlaw, Eq. 22) represents all clusters including disturbed ones.
- ad hoc to paper Simulated images without AGN are representative enough to quantify parameter biases and selection.
- domain assumption Catalogue R500 values are accurate enough for scaled morphological parameters.
- domain assumption Image-based eSASS detection in simulations approximates photon-based real detection.
- domain assumption Baryon fraction fB = 0.175 and mean mass per electron are adopted from external cosmology and plasma tables.
Cite this review
Pith. "Pith review of The SRG/eROSITA all-sky survey: The morphologies of clusters of galaxies I: A catalogue of morphological parameters." pith.science (2026). https://pith.science/paper/3EVGGUFT
@misc{pith2026250202239,
author = {Pith},
title = {Pith review of: The SRG/eROSITA all-sky survey: The morphologies of clusters of galaxies I: A catalogue of morphological parameters},
year = {2026},
howpublished = {\url{https://pith.science/paper/3EVGGUFT}},
note = {Machine review of arXiv:2502.02239}
}
read the original abstract
The first SRG/eROSITA all-sky X-ray survey, eRASS1, resulted in a catalogue of over twelve thousand optically-confirmed galaxy groups and clusters in the western Galactic hemisphere. Using the eROSITA images of these objects, we measure and study their morphological properties, including their concentration, central density and slope, ellipticity, power ratios, photon asymmetry, centroid shift and Gini coefficient. We also introduce new forward-modelled parameters which take account of the instrument point spread function (PSF), which are slosh, which measures how asymmetric the surface brightness distribution is, and multipole magnitudes, which are analogues to power ratios. Using simulations, we find some non forward-modelled parameters are strongly biased due to PSF and data quality. For the same clusters, we find similar values of concentration and central density compared to results by ourselves using Chandra and previous results from XMM-Newton. The population as a whole has log concentrations which are typically around 0.3 dex larger than South Pole Telescope or Planck-selected samples and the deeper eFEDS sample. The exposure time, detection likelihood threshold, extension likelihood threshold and number of counts affect the concentration distribution, but generally not enough to reduce the concentration to match the other samples. The concentration of clusters in the survey strongly affects whether they are detected as a function of redshift and luminosity. We introduce a combined disturbance score based on a Gaussian mixture model fit to several of the parameters. For brighter clusters, around 1/4 of objects are classified as disturbed using this score, which may be due to our sensitivity to concentrated objects.
Figures
Figures from the paper (20 more)
Forward citations
Cited by 1 Pith paper
-
Enhancing Galaxy Classification with U-Net Variational Autoencoders for Image Denoising
Applying a U-Net VAE denoising step to galaxy images before classification is reported to improve accuracy, reaching 97.45% with a GCNN on Galaxy10 DECaLS, although no direct noisy baseline is presented.
Reference graph
Works this paper leans on
-
[1]
G., van den Bergh, S., & Nair, P
Abraham, R. G., van den Bergh, S., & Nair, P. 2003, ApJ, 588, 218
2003
-
[2]
J., & Scott, P
Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481
2009
-
[3]
2016, ApJ, 827, 112 Bîrzan, L., Rafferty, D
Biffi, V ., Borgani, S., Murante, G., et al. 2016, ApJ, 827, 112 Bîrzan, L., Rafferty, D. A., McNamara, B. R., Wise, M. W., & Nulsen, P. E. J. 2004, ApJ, 607, 800
work page 2016
-
[4]
E., Stalder, B., de Haan, T., et al
Bleem, L. E., Stalder, B., de Haan, T., et al. 2015, ApJS, 216, 27 Böhringer, H., Pratt, G. W., Arnaud, M., et al. 2010, A&A, 514, A32 Böhringer, H., Schuecker, P., Pratt, G. W., et al. 2007, A&A, 469, 363 Böhringer, H. & Werner, N. 2010, A&A Rev., 18, 127
work page 2015
-
[5]
2022, A&A, 661, A1
Brunner, H., Liu, T., Lamer, G., et al. 2022, A&A, 661, A1
2022
-
[6]
2024, A&A, 685, A106
Bulbul, E., Liu, A., Kluge, M., et al. 2024, A&A, 685, A106
2024
-
[7]
2022, A&A, 661, A10
Bulbul, E., Liu, A., Pasini, T., et al. 2022, A&A, 661, A10
2022
-
[8]
Buote, D. A. & Tsai, J. C. 1995, ApJ, 452, 522
1995
Show all 53 references
-
[9]
G., Ettori, S., Lovisari, L., et al
Campitiello, M. G., Ettori, S., Lovisari, L., et al. 2022, A&A, 665, A117
2022
-
[10]
& Fusco-Femiano, R
Cavaliere, A. & Fusco-Femiano, R. 1978, A&A, 70, 677
1978
-
[11]
H., et al
Clowe, D., Bradaˇc, M., Gonzalez, A. H., et al. 2006, ApJ, 648, L109
2006
-
[12]
2020, The Open Journal of As- trophysics, 3, 13
Comparat, J., Eckert, D., Finoguenov, A., et al. 2020, The Open Journal of As- trophysics, 3, 13
2020
-
[13]
2011, A&A, 526, A79
Eckert, D., Molendi, S., & Paltani, S. 2011, A&A, 526, A79
2011
-
[14]
Edge, A. C. & Stewart, G. C. 1991, MNRAS, 252, 414
1991
-
[15]
Fabian, A. C. 2012, ARA&A, 50, 455
2012
-
[16]
W., Lang, D., & Goodman, J
Foreman-Mackey, D., Hogg, D. W., Lang, D., & Goodman, J. 2013, PASP, 125, 306
2013
-
[17]
E., Bulbul, E., et al
Ghirardini, V ., Bahar, Y . E., Bulbul, E., et al. 2022, A&A, 661, A12
2022
-
[18]
2023, MNRAS, 518, 4238
Gianfagna, G., Rasia, E., Cui, W., et al. 2023, MNRAS, 518, 4238
2023
-
[19]
S., Mittal, R., Reiprich, T
Hudson, D. S., Mittal, R., Reiprich, T. H., et al. 2010, A&A, 513, A37
2010
-
[20]
J., & Finoguenov, A
Johnson, R., Ponman, T. J., & Finoguenov, A. 2009, MNRAS, 395, 1287 Käfer, F., Finoguenov, A., Eckert, D., et al. 2020, A&A, 634, A8 Käfer, F., Finoguenov, A., Eckert, D., et al. 2019, A&A, 628, A43
2009
-
[21]
2024, A&A, 688, A210
Kluge, M., Comparat, J., Liu, A., et al. 2024, A&A, 688, A210
2024
-
[22]
T., Kravtsov, A
Lau, E. T., Kravtsov, A. V ., & Nagai, D. 2009, ApJ, 705, 1129
2009
-
[23]
T., Nagai, D., Kravtsov, A
Lau, E. T., Nagai, D., Kravtsov, A. V ., Vikhlinin, A., & Zentner, A. R. 2012, ApJ, 755, 116
2012
-
[24]
2024, A&A, 688, A186
Liu, A., Bulbul, E., Shin, T., et al. 2024, A&A, 688, A186
2024
-
[25]
M., Primack, J., & Madau, P
Lotz, J. M., Primack, J., & Madau, P. 2004, AJ, 128, 163
2004
-
[26]
R., Jones, C., et al
Lovisari, L., Forman, W. R., Jones, C., et al. 2017, ApJ, 846, 51
2017
-
[27]
Mann, A. W. & Ebeling, H. 2012, MNRAS, 420, 2120
2012
-
[28]
& Vikhlinin, A
Markevitch, M. & Vikhlinin, A. 2007, Phys. Rep., 443, 1
2007
-
[29]
W., Bayliss, M., et al
McDonald, M., Allen, S. W., Bayliss, M., et al. 2017, ApJ, 843, 28
2017
-
[30]
A., et al
McDonald, M., Bayliss, M., Benson, B. A., et al. 2012, Nature, 488, 349
2012
-
[31]
A., Vikhlinin, A., et al
McDonald, M., Benson, B. A., Vikhlinin, A., et al. 2013, ApJ, 774, 23
2013
-
[32]
McNamara, B. R. & Nulsen, P. E. J. 2012, New Journal of Physics, 14, 055023
2012
-
[33]
& Goulding, A
Melchior, P. & Goulding, A. D. 2018, Astronomy and Computing, 25, 183
2018
-
[34]
2024, A&A, 682, A34
Merloni, A., Lamer, G., Liu, T., et al. 2024, A&A, 682, A34
2024
-
[35]
H., & Jaritz, V
Mittal, R., Hicks, A., Reiprich, T. H., & Jaritz, V . 2011, A&A, 532, A133
2011
-
[36]
J., Evrard, A
Mohr, J. J., Evrard, A. E., Fabricant, D. G., & Geller, M. J. 1995, ApJ, 447, 8
1995
-
[37]
A., et al
Nurgaliev, D., McDonald, M., Benson, B. A., et al. 2013, ApJ, 779, 112 O’Hara, T. B., Mohr, J. J., Bialek, J. J., & Evrard, A. E. 2006, ApJ, 639, 64
2013
-
[38]
K., Fabian, A
Panagoulia, E. K., Fabian, A. C., & Sanders, J. S. 2014, MNRAS, 438, 2341
2014
-
[39]
B., Fardal, M
Poole, G. B., Fardal, M. A., Babul, A., et al. 2006, MNRAS, 373, 881
2006
-
[40]
W., Croston, J
Pratt, G. W., Croston, J. H., Arnaud, M., & Böhringer, H. 2009, A&A, 498, 361
2009
-
[41]
2021, A&A, 647, A1
Predehl, P., Andritschke, R., Arefiev, V ., et al. 2021, A&A, 647, A1
2021
-
[42]
A., McNamara, B
Rafferty, D. A., McNamara, B. R., Nulsen, P. E. J., & Wise, M. W. 2006, ApJ, 652, 216
2006
-
[43]
2013, The Astronomical Review, 8, 40
Rasia, E., Meneghetti, M., & Ettori, S. 2013, The Astronomical Review, 8, 40
2013
-
[44]
2017, MNRAS, 468, 1917
Rossetti, M., Gastaldello, F., Eckert, D., et al. 2017, MNRAS, 468, 1917
2017
-
[45]
S., Fabian, A
Sanders, J. S., Fabian, A. C., Russell, H. R., & Walker, S. A. 2018, MNRAS, 474, 1065
2018
-
[46]
S., Rosati, P., Tozzi, P., et al
Santos, J. S., Rosati, P., Tozzi, P., et al. 2008, A&A, 483, 35
2008
-
[47]
2022, A&A, 665, A78
Seppi, R., Comparat, J., Bulbul, E., et al. 2022, A&A, 665, A78
2022
-
[48]
N., Bean, R., Doré, O., et al
Spergel, D. N., Bean, R., Doré, O., et al. 2007, ApJS, 170, 377
2007
-
[49]
& Sarazin, C
Valdarnini, R. & Sarazin, C. L. 2021, MNRAS, 504, 5409
2021
-
[50]
R., et al
Vikhlinin, A., Burenin, R., Forman, W. R., et al. 2007, in Heating versus Cool- ing in Galaxies and Clusters of Galaxies, ed. H. Böhringer, G. W. Pratt, A. Finoguenov, & P. Schuecker, 48
2007
-
[51]
A., Ebeling, H., et al
Vikhlinin, A., Burenin, R. A., Ebeling, H., et al. 2009, ApJ, 692, 1033 V oit, G. M. 2005, Reviews of Modern Physics, 77, 207
2009
-
[52]
H., Bullock, J
Wechsler, R. H., Bullock, J. S., Primack, J. R., Kravtsov, A. V ., & Dekel, A. 2002, ApJ, 568, 52 Weißmann, A., Böhringer, H., Šuhada, R., & Ameglio, S. 2013, A&A, 549, A19
2002
-
[53]
E., Pacaud, F., Reiprich, T
Xu, W., Ramos-Ceja, M. E., Pacaud, F., Reiprich, T. H., & Erben, T. 2022, A&A, 658, A59 Article number, page 31 of 32 A&A proofs: manuscript no. morph_cat Appendix A: Description of catalogue Table A.1 describes the columns in the morphology catalogue, giving the column name a...
2022
Reviewed August 9, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.