REVIEW 4 major objections 5 minor 54 references
Measuring satellite galaxy subhalo masses in redMaPPer clusters with UNIONS weak lensing data
T0 review · 4 major / 5 minor · reviewed 2026-08-02 · deepseek-v4-flash
Pith's one-line read This paper shows that satellite galaxies in clusters lose dark matter as they fall inward: the subhalo-to-stellar mass ratio rises from about 12 near the core to 53 in the outskirts.
desk verdict A large-sample UNIONS measurement confirms the tidal-stripping trend in direction, but the fitting model is formally rejected by the data, so the quoted HSMR amplitudes are model-dependent. 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 instrument is the combined excess surface density model ΔΣ(R) = A·ΔΣ_host + ΔΣ_sub + ΔΣ_*, applied to stacked lensing. The host-halo term is generated by Monte-Carlo point masses following an NFW profile centred on a brightest cluster galaxy whose offset from the catalogue position is drawn from a Rayleigh distribution with a fitted scale σ; the subhalo term is a projected NFW profile with free mass M_sub; the stellar term is a point mass. This decomposition isolates the subhalo's own lensing contribution from the dominant cluster halo, which is what allows a mass to be assigned to the satellite halos.
What would settle it
Re-fit the same lensing signal for the outer bins using cluster centres from X-ray observations or including a two-halo term; if the subhalo-to-stellar mass ratios in those bins drop to the inner-bin value, the reported tidal-stripping trend is an artifact of the offset-host model. The paper's own alternative two-centre host model gives ratios of 4, 13, and 32, so the trend's amplitude is sensitive to host-offset modelling.
Extended reading notes
Core claim
The paper's central claim is that the dark-matter halos of cluster satellites are progressively stripped toward the cluster centre, and that this can be measured cleanly in stacked galaxy-galaxy lensing. From the excess surface density signal it infers median subhalo masses log M_sub/M_sun = 12.02, 12.45, and 12.58 in the 0.143-0.43, 0.43-0.86, and 0.86-1.29 Mpc bins, giving subhalo-to-stellar mass ratios of 11.99, 33.68, and 53.02. The monotonic increase is the result the paper wants to be judged on; it is presented as confirmation that tidal stripping or similar environmental processes remove dark matter from infalling galaxies while their stellar mass remains intact.
Load-bearing premise
The host-halo subtraction assumes the true cluster centre is offset from the catalogued brightest cluster galaxy by a single Rayleigh distribution with a fitted scale per bin; the paper itself notes (Section 6) that this scale disagrees with X-ray-based estimates in the outer bins and that a two-halo term is omitted beyond roughly 0.6 Mpc. If the offset model is wrong, the inferred subhalo masses and the radial trend are biased.
Editorial extensions
If this is right
- If the trend is real, an inner satellite carries roughly a quarter of the dark-matter-to-stellar mass ratio of an outer satellite, quantifying how much dark matter cluster tides remove.
- The result confirms the standard hierarchical picture: satellite galaxies' dark halos are stripped preferentially near the cluster centre while their stellar mass stays intact.
- With roughly 330,000 lens satellites and 4.5 million source galaxies, the inner-bin measurement is far tighter than earlier work and anchors the radial subhalo-to-stellar mass relation at low radius.
Reading between the lines
- Inference: a natural next split is by satellite stellar mass or host richness; if stripping efficiency depends on mass, low-mass satellites should show a steeper HSMR trend, testable with the same lens-source pairs.
- Inference: the fitted Rayleigh σ growing with cluster-centric radius may be absorbing a missing two-halo term or an anisotropic mis-centring distribution rather than a true physical offset; comparing with X-ray-centred cluster samples would discriminate.
- Inference: if the trend survives such tests, stellar-mass-based halo estimates for cluster members are environmentally biased, and cluster-centric radius should enter abundance-matching calibrations for satellites.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper measures the galaxy-galaxy lensing signal (excess surface density) around ~330,000 redMaPPer satellite galaxies in three cluster-centric radius bins, using UNIONS/ShapePipe source shapes. The lensing signal is modelled as the sum of a point-mass stellar term, an NFW subhalo term with free subhalo mass M_sub, and an offset NFW host-halo term with a free amplitude A and a Rayleigh-distributed BCG mis-centring scale sigma. The key result is a claimed strong positive trend in subhalo-to-stellar mass ratio (HSMR) with cluster-centric radius: M_sub/M* = 11.99, 33.68, 53.02 in the three bins, interpreted as observational confirmation of tidal stripping of satellite dark matter halos. The paper also presents an alternative 'admixture' host-halo model in Appendix B that fits the data better and yields lower HSMR values (4.0, 13.4, 32.1), while preserving the trend.
Significance. If the result holds, the paper would provide a high-S/N observational confirmation of tidal stripping of satellite subhalos in clusters, using a large and valuable data combination (UNIONS sources, redMaPPer lenses). The data volume is a genuine strength: ~4.5 million source galaxies and ~330,000 satellite lenses. The authors are transparent about their modelling choices and include an appendix with an alternative host-halo model. The trend with cluster-centric radius is plausible and broadly consistent with earlier work. However, the significance of the paper is undermined by the fact that the adopted model is statistically rejected by the data (reduced chi-square 3.59, 2.77, 3.39 for 11 dof in Table 1), and by the strong model-dependence of the HSMR values (Appendix B). The conclusion 'confirm tidal stripping' is therefore not yet supported at the level claimed.
major comments (4)
- [§5, Table 1] The adopted model is statistically rejected by the data. The reduced chi-square values are 3.59, 2.77, and 3.39 for 11 degrees of freedom in the three bins, corresponding to very small p-values. This contradicts the abstract's claim that the model 'effectively reproduces the observed lensing signal'. The paper itself documents large residual features, including an unexplained 'hump' near 0.5 Mpc in the outermost bin (Section 6, Figure 4). Since the central claim is based on parameters derived from this model, the poor fit is a load-bearing problem: the quoted M_sub and HSMR values are derived from a model that does not describe the data.
- [§2.2, Eq. (6), Table 2] The mis-centring model is a single Rayleigh distribution with a free scale sigma, fitted independently in each bin. The best-fit sigma increases from 83.5 to 347.7 kpc across the three bins, and the outer two bins are inconsistent with the external Zhang et al. (2019) estimates by factors of ~1.6 and ~4 (Table 2). A large sigma smooths the negative host-halo trough and can be partially degenerate with a larger M_sub, especially in the outer bin. The authors acknowledge the divergence but do not quantify how much of the HSMR trend is driven by this freedom. A concrete test would be to impose a prior on sigma from Zhang et al. (2019) or a more flexible mis-centring distribution and show the effect on M_sub.
- [Appendix B, Table B1] The paper's own alternative 'admixture' model fits the data substantially better (reduced chi-square 1.72, 1.17, 1.76) and yields HSMR values (4.0, 13.4, 32.1) that are a factor of ~1.6–3 lower than the primary model (11.99, 33.68, 53.02) in the corresponding bins. This demonstrates that the reported HSMR amplitudes—and by extension the quantitative strength of the 'strong positive trend'—are strongly model-dependent. The trend itself persists in the admixture model, but the paper does not provide a model-comparison or a formal test of whether the trend is significant under both models. The central claim of a 'strong' trend is therefore not robust to reasonable changes in the host-halo treatment.
- [§2, §6] The two-halo term is omitted but is expected to be non-negligible beyond ~0.6 Mpc, as the authors state in Section 2 and discuss in Section 6. The outermost cluster-centric bin (0.86–1.29 Mpc) is exactly where such a term matters. An omitted positive large-scale contribution can bias the fitted host-halo amplitude A and, through degeneracy with M_sub, bias the outer-bin HSMR. Since the two-halo term is a known, physical contribution rather than a nuisance, the analysis should either include it or restrict the fitting range to scales where it is negligible. At minimum, the paper should quantify the systematic shift in M_sub and HSMR when a two-halo term is added.
minor comments (5)
- [Abstract] The phrase 'confirm tidal stripping' is too strong given the modelling caveats; the analysis shows consistency with tidal stripping, not a direct confirmation. Consider softening to 'are consistent with'.
- [§1] Typo: 'partially loosing mass' should be 'losing mass'.
- [§2.1] Typo: 'the former is is easier'.
- [§6] In the discussion of the common-halo test, the sigma values are reported with asymmetric errors that are not all in the same convention; ensure consistency and state the median and 16th/84th percentiles throughout.
- [Table 1] The column header 'Pmem' is described in the caption as 'median cluster membership probability', but the text in Section 3.1 refers to a threshold of 0.8. Clarify the definition and whether the values in the table are medians of the lens sample.
Circularity Check
No circularity: the HSMR trend is a fitted measurement, not a prediction forced by construction.
full rationale
The derivation chain is: measure ΔΣ(R) from UNIONS/redMaPPer lens-source pairs; model it as Eq. (3), ΔΣ(R) = A·ΔΣ_host + ΔΣ_sub + ΔΣ_*; fit M_sub, A, and the Rayleigh mis-centering scale σ per radial bin by MCMC; then form HSMR = M_sub/M_* using independent stellar masses from Zou et al. (2019). No stage defines a target quantity in terms of itself: M_sub is obtained from the lensing signal, not from the stated trend, and the trend is an interpretation of the fitted masses. The host-halo term is precomputed from the richness–mass relation and redMaPPer BCG positions, and the stellar term uses external stellar masses; neither is redefined to force the HSMR trend. The mis-centering Rayleigh form is adopted from external Kumar & More (2024) and compared with external Zhang et al. (2019). The high reduced χ² and the Appendix B admixture model show model dependence and possible bias, but model-dependence is not circularity: the alternative model still yields a radial trend and the main result is not an identity. Self-citations (UNIONS survey, ShapePipe, Cheng et al. 2025 n(z) calibration) are data/methodology sources, not uniqueness theorems or ansatz smuggled in to force the conclusion. Thus no circular step is evident, and the paper is best treated as a self-contained measurement with model-systematics caveats.
Assumptions & free parameters
free parameters (3)
- log10(Msub/Msun) per cluster-centric bin =
12.02, 12.45, 12.58
- Host halo rescaling factor A per bin =
0.66, 0.77, 1.19
- Rayleigh BCG mis-centering scale sigma per bin =
83.5, 132.7, 347.7 kpc
assumptions (8)
- domain assumption NFW profile describes both cluster host halos and satellite subhalos
- domain assumption Mass-concentration relation from Duffy et al. (2008)
- domain assumption Mass-richness relation from Rykoff et al. (2012) (Eq. 5)
- domain assumption Flat ΛCDM cosmology with H0=70 km/s/Mpc, OmegaM=0.3, sigma8=0.81, ns=0.95
- ad hoc to paper BCG mis-centering follows a single Rayleigh distribution with free sigma (Eq. 6)
- domain assumption Cluster photometric redshift assigned to satellites without spectroscopic redshift
- domain assumption No two-halo term is included for satellites
- domain assumption Membership probability threshold Pmem > 0.8 ensures clean satellite selection
Cite this review
Pith. "Pith review of Measuring satellite galaxy subhalo masses in redMaPPer clusters with UNIONS weak lensing data." pith.science (2026). https://pith.science/paper/QGJ7FFEN
@misc{pith2026260714207,
author = {Pith},
title = {Pith review of: Measuring satellite galaxy subhalo masses in redMaPPer clusters with UNIONS weak lensing data},
year = {2026},
howpublished = {\url{https://pith.science/paper/QGJ7FFEN}},
note = {Machine review of arXiv:2607.14207}
}
read the original abstract
In this work, we present the observed galaxy-galaxy lensing signal of satellite galaxies in rich clusters obtained from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). Such observations are crucial for understanding dark matter halo physics and interaction dynamics in the cluster environment. Our goal is to investigate the tidal stripping of cluster subhalos. Theoretical predictions, supported by recent studies, suggest that as satellite galaxies fall into clusters, their dark matter halos are stripped and dispersed into the cluster's main halo, while their stellar mass remains relatively intact. A robust method to probe this phenomenon is through statistical measurements of the excess surface mass density of satellite galaxies at different cluster-centric distances. Our results reveal a significant lensing signal, with strong statistical power from the large number of lens-source pairs. A simple model used in the fitting process to constrain subhalo masses effectively reproduces the observed lensing signal at relevant satellite-centric distances. However, it lacks the complexity to accurately fit the excess surface density at larger distances, where the host cluster halo dominates. We measure the subhalo-to-stellar mass ratio as a function of cluster-centric radius and find a strong positive trend. These results confirm tidal stripping or other environmental processes that impact the relative masses of a satellite galaxy's stellar component and its dark matter halo.
Figures
Figures from the paper (3 more)
Reference graph
Works this paper leans on
-
[1]
Measuring subhalo mass in redMaPPer clusters with CFHT Stripe 82 Survey. , keywords =. doi:10.1093/mnras/stw494 , archivePrefix =. 1507.01464 , primaryClass =
-
[2]
Assessing mass-loss and stellar-to-halo mass ratio of satellite galaxies: a galaxy-galaxy lensing approach utilizing DECaLS DR8 data. , keywords =. doi:10.1093/mnras/stae121 , archivePrefix =. 2305.13694 , primaryClass =
-
[5]
Gao, L. and White, S. D. M. and Jenkins, A. and Stoehr, F. and Springel, V. , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2004 , month =. doi:10.1111/j.1365-2966.2004.08360.x , url =
arXiv 2004
-
[6]
Contini, E. and De Lucia, G. and Villalobos, A. and Borgani, S. , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2013 , month =. doi:10.1093/mnras/stt2174 , url =
-
[7]
Monthly Notices of the Royal Astronomical Society , volume =
van den Bosch, Frank C and Ogiya, Go and Hahn, Oliver and Burkert, Andreas , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2017 , month =. doi:10.1093/mnras/stx2956 , url =
-
[8]
Anthony R. Pullen and Andrew J. Benson and Leonidas A. Moustakas , title =. The Astrophysical Journal , abstract =. 2014 , month =. doi:10.1088/0004-637X/792/1/24 , url =
-
[9]
Du, Xiaolong and Behrens, Christoph and Niemeyer, Jens C. , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2016 , month =. doi:10.1093/mnras/stw2724 , url =
-
[11]
Springel, Volker and White, Simon D. M. and Tormen, Giuseppe and Kauffmann, Guinevere , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2001 , month =. doi:10.1046/j.1365-8711.2001.04912.x , url =
arXiv 2001
Show all 54 references
-
[12]
Bower, R. G. and Benson, A. J. and Malbon, R. and Helly, J. C. and Frenk, C. S. and Baugh, C. M. and Cole, S. and Lacey, C. G. , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2006 , month =. doi:10.1111/j.1365-2966.2006.10519.x , url =
2006
-
[13]
Monthly Notices of the Royal Astronomical Society , volume =
Xie, Lizhi and Gao, Liang , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2015 , month =. doi:10.1093/mnras/stv2077 , url =
2015 doi
-
[14]
Evidence for substructure in lens galaxies?
Mao, Shu-de and Schneider, Peter. Evidence for substructure in lens galaxies?. Mon. Not. Roy. Astron. Soc. 1998. doi:10.1046/j.1365-8711.1998.01319.x. arXiv:astro-ph/9707187
1998
-
[15]
Benton Metcalf and Piero Madau , title =
R. Benton Metcalf and Piero Madau , title =. The Astrophysical Journal , abstract =. 2001 , month =. doi:10.1086/323695 , url =
2001 doi
-
[16]
Ostriker and Jochen Weller , title =
Shude Mao and Yipeng Jing and Jeremiah P. Ostriker and Jochen Weller , title =. The Astrophysical Journal , abstract =. 2004 , month =. doi:10.1086/383413 , url =
2004 doi
-
[17]
Xu, D. D. and Mao, Shude and Wang, Jie and Springel, V. and Gao, Liang and White, S. D. M. and Frenk, Carlos S. and Jenkins, Adrian and Li, Guoliang and Navarro, Julio F. , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2009 , month =. doi:10.1111/j.1...
2009
-
[18]
Nierenberg, A. M. and Treu, T. and Wright, S. A. and Fassnacht, C. D. and Auger, M. W. , title = ". Monthly Notices of the Royal Astronomical Society , volume =. 2014 , month =. doi:10.1093/mnras/stu862 , url =
2014 doi
-
[19]
The Astrophysical Journal , volume=
Hubble Space Telescope observations of the lensing cluster Abell 2218 , author=. The Astrophysical Journal , volume=. 1996 , publisher=
1996
-
[20]
The Astronomy and Astrophysics Review , volume=
Cluster lenses , author=. The Astronomy and Astrophysics Review , volume=. 2011 , publisher=
2011
-
[21]
Monthly Notices of the Royal Astronomical Society , volume=
Gravitational imaging of cold dark matter substructures , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2005 , publisher=
2005
-
[22]
Monthly Notices of the Royal Astronomical Society , volume=
Detection of a dark substructure through gravitational imaging , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2010 , publisher=
2010
-
[23]
nature , volume=
Gravitational detection of a low-mass dark satellite galaxy at cosmological distance , author=. nature , volume=. 2012 , publisher=
2012
-
[24]
Monthly Notices of the Royal Astronomical Society , volume=
First galaxy--galaxy lensing measurement of satellite halo mass in the CFHT Stripe-82 Survey , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2014 , publisher=
2014
-
[25]
Monthly Notices of the Royal Astronomical Society , volume=
The masses of satellites in GAMA galaxy groups from 100 square degrees of KiDS weak lensing data , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2015 , publisher=
2015
-
[26]
Monthly Notices of the Royal Astronomical Society , volume=
The galaxy--subhalo connection in low-redshift galaxy clusters from weak gravitational lensing , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2018 , publisher=
2018
-
[27]
Monthly Notices of the Royal Astronomical Society , volume=
The stellar-to-halo mass relation of GAMA galaxies from 100 deg2 of KiDS weak lensing data , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2016 , publisher=
2016
-
[28]
Monthly Notices of the Royal Astronomical Society , volume=
Stellar-to-halo mass relation of cluster galaxies , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2017 , publisher=
2017
-
[29]
Astronomy & Astrophysics , volume=
KiDS+ GAMA: The weak lensing calibrated stellar-to-halo mass relation of central and satellite galaxies , author=. Astronomy & Astrophysics , volume=. 2020 , publisher=
2020
-
[30]
Astronomy & Astrophysics , volume=
ShapePipe: A new shape measurement pipeline and weak-lensing application to UNIONS/CFIS data , author=. Astronomy & Astrophysics , volume=. 2022 , publisher=
2022
-
[31]
The Astrophysical Journal Supplement Series , volume=
The RedMaPPer galaxy cluster catalog from DES science verification data , author=. The Astrophysical Journal Supplement Series , volume=. 2016 , publisher=
2016
-
[32]
Publications of the Astronomical Society of Australia , volume=
Damn you, little h!(or, real-world applications of the hubble constant using observed and simulated data) , author=. Publications of the Astronomical Society of Australia , volume=. 2013 , publisher=
2013
-
[33]
Monthly Notices of the Royal Astronomical Society , volume=
The assembly of galaxies in a hierarchically clustering universe , author=. Monthly Notices of the Royal Astronomical Society , volume=. 1995 , publisher=
1995
-
[34]
Monthly Notices of the Royal Astronomical Society: Letters , volume=
Dark matter halo concentrations in the Wilkinson Microwave Anisotropy Probe year 5 cosmology , author=. Monthly Notices of the Royal Astronomical Society: Letters , volume=. 2008 , publisher=
2008
-
[35]
The Astrophysical Journal Supplement Series , volume=
COLOSSUS: A python toolkit for cosmology, large-scale structure, and dark matter halos , author=. The Astrophysical Journal Supplement Series , volume=. 2018 , publisher=
2018
-
[36]
The Astrophysical Journal Letters , volume=
The Atacama Cosmology Telescope: weighing distant clusters with the most ancient light , author=. The Astrophysical Journal Letters , volume=. 2020 , publisher=
2020
-
[37]
The Astrophysical Journal Supplement Series , volume=
The eighth data release of the Sloan Digital Sky Survey: first data from SDSS-III , author=. The Astrophysical Journal Supplement Series , volume=. 2011 , publisher=
2011
-
[38]
2016 , journal=
The DESI experiment part I: science, targeting, and survey design , author=. 2016 , journal=
2016
-
[39]
Astrophysics Source Code Library , pages=
dsigma: Galaxy-galaxy lensing Python package , author=. Astrophysics Source Code Library , pages=
-
[40]
Unbiased estimation of the inverse covariance matrix , author=
Why your model parameter confidences might be too optimistic. Unbiased estimation of the inverse covariance matrix , author=. Astronomy & Astrophysics , volume=. 2007 , publisher=
2007
-
[41]
Journal of Cosmology and Astroparticle Physics , volume=
Analytic models of plausible gravitational lens potentials , author=. Journal of Cosmology and Astroparticle Physics , volume=. 2009 , publisher=
2009
-
[42]
redmapper. i. algorithm and sdss dr8 catalog , author=. The Astrophysical Journal , volume=. 2014 , publisher=
2014
-
[43]
, keywords =
Photometric Redshifts and Stellar Masses for Galaxies from the DESI Legacy Imaging Surveys. , keywords =. doi:10.3847/1538-4365/ab1847 , adsurl =
-
[44]
Publications of the Astronomical Society of the Pacific , volume=
emcee: the MCMC hammer , author=. Publications of the Astronomical Society of the Pacific , volume=. 2013 , publisher=
2013
-
[45]
The Astronomical Journal , volume=
Sdss-iii: Massive spectroscopic surveys of the distant universe, the milky way, and extra-solar planetary systems , author=. The Astronomical Journal , volume=. 2011 , publisher=
2011
-
[46]
arXiv preprint arXiv:1612.05560 , year=
The pan-starrs1 surveys , author=. arXiv preprint arXiv:1612.05560 , year=
-
[47]
The Astronomical Journal , volume=
Redshift measurement and spectral classification for eboss galaxies with the redmonster software , author=. The Astronomical Journal , volume=. 2016 , publisher=
2016
-
[48]
2024 , eprint=
Environmental dependence on galaxy-halo connections for satellites using HSC weak lensing , author=. 2024 , eprint=
2024
-
[49]
, keywords =
Core condensation in heavy halos: a two-stage theory for galaxy formation and clustering. , keywords =. doi:10.1093/mnras/183.3.341 , adsurl =
- [50]
-
[51]
, keywords =
Robust Optical Richness Estimation with Reduced Scatter. , keywords =. doi:10.1088/0004-637X/746/2/178 , archivePrefix =. 1104.2089 , primaryClass =
-
[52]
, keywords =
Black Hole Halo Mass Relation from UNIONS Weak Lensing. , keywords =. doi:10.3847/2041-8213/ad58b0 , archivePrefix =. 2402.10740 , primaryClass =
- [53]
-
[54]
, keywords =
The Dark Energy Survey: more than dark energy - an overview. , keywords =. doi:10.1093/mnras/stw641 , archivePrefix =. 1601.00329 , primaryClass =
- [55]
-
[56]
, keywords =
On the measurements of assembly bias and splashback radius using optically selected galaxy clusters. , keywords =. doi:10.1093/mnras/stz2832 , archivePrefix =. 1905.07557 , primaryClass =
1905 arXiv
-
[57]
Monthly Notices of the Royal Astronomical Society , volume=
Dark Energy Surveyed Year 1 results: calibration of cluster mis-centring in the redMaPPer catalogues , author=. Monthly Notices of the Royal Astronomical Society , volume=. 2019 , publisher=
2019
Reviewed August 2, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.