REVIEW 3 major objections 6 minor 58 references
Dynamical analysis of stacked samples of asymmetric, non-static, self-gravitating systems
T0 review · 3 major / 6 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read The paper claims that multiplying a stacked-cluster Jeans mass estimate by a product of two correction factors, $F_1$ and $F_2$, recovers the true mean mass profile without bias, correcting a typical 20 percent overestimate at the virial…
desk verdict A careful simulation study that identifies a real ~20% systematic in stacked dynamical masses and offers a workable correction, though the transfer to observed galaxies is the main caveat. 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 object is the generalized Jeans equation for velocities measured in the free-falling rest frame of the cluster centre: $\partial_t n\langle v_i\rangle + \partial_j n\langle v_i v_j\rangle + n a_{0i} + n\partial_i \Phi = 0$. From it the paper forms the mass estimator $M(<r) = -r^2\tilde{g}(r) F_1(r)F_2(r)/G$. The factor $F_1 = 1 + \hat{r}_i\partial_t n\langle v_i\rangle / (\hat{r}_i\partial_j n\langle v_i v_j\rangle)$ corrects for the neglected rate of change of momentum density, i.e. infall; it is evaluated by differencing two simulation outputs separated in time. The factor $F_2$ is the ratio of the isotropically surface-averaged radial gravity to the galaxy-density-weighted radial gravity, absorbing both the nonsphericity of individual haloes and the non-zero acceleration $a_0$ of the centre; it is evaluated with spherical grids of massless test particles placed around each halo.
What would settle it
Take a real stacked cluster sample with measured velocity dispersions and weak-lensing masses; if, after applying the simulation-calibrated $F_1F_2$ correction, the ratio of dynamical mass to lensing mass at $r_{200}$ differs from unity by more than the combined measurement error, then the correction factors are incomplete or the freely-falling-centre assumption fails. A complementary simulation test is to rerun the same stacking analysis in a hydrodynamical simulation where the central galaxy experiences dynamical friction and baryonic feedback; a residual bias in the corrected mass profile would show that equation (3) needs an extra term.
Extended reading notes
Core claim
The central discovery is that the usual Jeans-equation mass estimate of a stacked cluster sample is biased high because two terms are neglected: the time-rate of change of radial momentum density, captured by $F_1$, and the difference between density-weighted and surface-averaged gravity, which includes the acceleration of the cluster centre, captured by $F_2$. The paper derives the corrected mass formula $M(<r) = -r^2 \tilde{g}(r) F_1(r) F_2(r)/G$ from the generalized Jeans equation $\partial_t n\langle v_i\rangle + \partial_j n\langle v_i v_j\rangle + n a_{0i} + n\partial_i \Phi = 0$, where $a_0$ is the acceleration of the freely falling centre. In the simulation, applying $F_1F_2$ to the pressure-gradient estimate brings it onto the true mean mass profile at all radii shown. The two effects are distinct: $F_1$ matters mainly outside the virial radius and grows with halo mass, while $F_2$ is already non-negligible inside $r_{200}$ and is amplified when the stacked sample contains a wide range of halo masses.
Load-bearing premise
The load-bearing premise is that the observable cluster centre---the most bound particle of the most massive subhalo, averaged over a $30\,h^{-1}\,\mathrm{kpc}$ core---is a freely falling reference frame and that galaxies behave as collisionless dark-matter particles; if real brightest cluster galaxies are dragged by dynamical friction or feedback, or if galaxies are biased tracers of the velocity field, the generalized Jeans equation no longer describes the stack and the calibrated $F_1F_2$ values will not transfer to observations.
Editorial extensions
If this is right
- Stacked dynamical mass profiles from surveys should be multiplied by $F_1(r)F_2(r)$ before comparison with weak-lensing profiles; in the simulation this restores the true mean mass profile.
- The uncorrected Jeans mass is biased high by about 20 percent at $r_{200}$ for narrow mass-selected samples, and by up to a factor of two for cumulative samples selected above a velocity-dispersion threshold.
- Because $F_1$ grows with halo mass, the bias depends on sample selection; a mass estimate quoted without the stacking selection is incomplete.
- Rescaling individual clusters by their velocity dispersions before stacking would largely remove the wide-mass-range part of the bias, as the paper notes for cumulative samples.
Reading between the lines
- Inference: the same two-correction scheme should apply to other stacked dynamical probes such as caustic or virial-theorem mass estimates, but the numerical values of $F_1$ and $F_2$ would need to be re-calibrated for each estimator.
- Inference: applied to real survey stacks, the correction factors cannot be measured directly and must be taken from simulations, so the calibration inherits any cosmological-model dependence of the simulated halo population; mismatched concentrations would shift the correction.
- Inference: the gravitational-redshift signal in stacked clusters is sensitive to the same centre acceleration and density weighting, so it may carry a similar mass-dependent bias that could be isolated with the $F_2$-style weighting comparison.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper investigates biases in dynamical mass estimates of stacked galaxy clusters obtained by applying the Jeans equation to the stacked pressure tensor. The authors derive two correction factors: F1 (equation 7), which accounts for the time derivative of the radial momentum density, and F2 (equation 8), which accounts for the acceleration of the cluster centre and for the difference between density-weighted and solid-angle-weighted gravity. Using the Millennium simulation, they stack tens of thousands of haloes in narrow mass bins and in cumulative velocity-dispersion-selected samples, measure the pressure tensor and the correction terms, and report that the naive Jeans estimate is biased high by about 20% at the virial radius and by up to a factor of two for broad mass-range samples. After applying F1F2, the estimated mass profiles match the true mean mass profiles (Figure 1).
Significance. The decomposition of the bias into an out-of-equilibrium term F1 and a weighting/centre-acceleration term F2 is clear and useful. Equations (3)-(8) are derived carefully, and the numerical evaluation in the simulation is a direct measurement of phase-space quantities rather than a fit to the true mass profile, with no free parameters beyond the core radius and the time step. The result that stacked dynamical masses are biased high by tens of percent, with a strong dependence on sample breadth, is important for upcoming cluster surveys and for comparisons of dynamical masses with weak-lensing masses. However, because F2 is defined so that F1F2 algebraically recovers the true mass, the agreement in Figure 1 is a consistency check rather than an independent validation. The principal quantitative content is the measured bias factor 1/(F1F2) and its dependence on sample selection, and those estimates currently lack error bars and an explicit test of the galaxy-tracer assumption.
major comments (3)
- [Section 2, Eq. (8); Section 3.3] Equation (8) defines F2 as the ratio of the true enclosed mass to the density-weighted radial acceleration, so the product F1F2 recovers M(<r) by construction when the individual terms are measured exactly. The dotted curves in Figure 1 therefore verify the numerical implementation but do not independently confirm the physical model of the bias. The statement in Section 3.3 that the recovery is 'numerical justification of our reasoning' should be tempered, and the primary result should be presented as the measured bias factor 1/(F1F2) and its sample dependence.
- [Section 3 and Appendix A2] No uncertainties are reported for F1, F2, or the inferred bias factors. F1 is estimated from a single finite difference between a=1 and a=1.04, and F2 is described in Section 3.2 as 'quite noisy' beyond 3r200. Since the central quantitative claims (for example, the ~20% bias at r200 and the factor-of-two bias for cumulative samples) are calibrations intended for observational use, the paper should provide bootstrap or jackknife errors over haloes, and a convergence check on the time step Δa and on the angular binning used in Appendix A3.
- [Section 1, footnote 1; Appendix A1] The correction factors are evaluated from dark matter particles, with the most bound particle plus a 30 h^-1 kpc core average used as a proxy for a freely falling BCG. The abstract and Section 4 state that the bias can be used to correct dynamical mass estimates in surveys, but real galaxy tracers are not necessarily collisionless dark matter particles: galaxies can have a velocity bias relative to dark matter, and BCGs may be affected by dynamical friction or feedback-induced offsets. The transfer of F1F2 from this simulation to an observed cluster stack is therefore an untested assumption. The applicability claim should either be restricted to dark-matter tracers or the calibration should be repeated with subhalo or galaxy tracers.
minor comments (6)
- [Footnote 1] The footnote ends with the truncated phrase 'between the m.'; it should be completed (for example, 'between the mass distribution and the galaxies').
- [Title and Introduction] The title contains a stray space in 'non-s tatic', and the Introduction contains the typo 'deliever'. Both should be corrected.
- [Figure 1 caption] The caption uses the typos 'Mesti' and 'Mcorct'; these should be written as, for example, 'M_est' and 'M_corct'.
- [Appendix A2] Appendix A2 refers to '30 h^-1 Mpc cores', but the core radius defined in Appendix A1 is 30 h^-1 kpc; this appears to be a unit error.
- [Section 3.3] The text mixes units by quoting r < 0.1 h^-1 Mpc in one place and r200 in nearby statements; please define the normalization consistently throughout.
- [Acknowledgments] The acknowledgments contain 'a part for the first one', which should presumably read 'apart from', and 'we have made necessary revision', which should be plural.
Circularity Check
The F1F2 corrected-mass recovery in Fig. 1 is a definitional identity because F2's numerator is the true mass, but the paper's main bias estimate remains an independent simulation measurement; overall circularity is low.
-
self definitional
[Eq. (8), Sec. 3.2; Sec. 3.3, Fig. 1]
"The second is F2(r) = ∫ r̂_i[g_i(r) − a0_i]dΩ / [\bar n(r)^{-1} ∫ r̂_i[g_i(r) − a0_i]n(r)dΩ], (8) ... So by definition, the numerator is the same as −M(<r)/r^2, where M is now the averaged mass of the stacked cluster. ... Finally, we show with dotted lines in Fig. 1 that applying the correction term F1F2 does recover the true mass without any bias."
Because F2 is defined with the true stacked mass M(<r) in its numerator, equation (6) is an algebraic rearrangement of equation (5) using equation (4): the product F1F2 converts the density-weighted pressure-gradient estimate into the solid-angle-averaged gravitational acceleration whose Gauss-law integral is M(<r) by definition. The recovery of the true mass in Fig. 1 is therefore a consistency check built into the construction, not an independent numerical prediction. This does not make the measured bias 1/(F1F2) or the 20 percent overestimate circular, since those are direct simulation measurements comparing the uncorrected Jeans estimate with the simulated true mass.
full rationale
The derivation of equations (3)-(8) is self-contained. F1 is constructed from the pressure tensor and the time derivative of the radial momentum density, and F2 from the simulated acceleration field and the central acceleration; neither is fitted to the mass profile. The central claim, that uncorrected Jeans estimates are biased high by about 20 percent at r200 and by up to a factor of two for cumulative samples, is an independent measurement from the simulation's phase-space data. The self-citation to Cai et al. 2017 is only a 'see also' in a qualitative discussion of neighbouring haloes and is not load-bearing. The one genuine reduction is the validation in Fig. 1: since F2's numerator is defined to be -M(<r)/r^2, the corrected profile must equal the true profile by construction. This is a mild self-definitional step rather than a fitted-input prediction, and it does not undermine the independent content of the bias estimate, so the circularity score is low.
Assumptions & free parameters
free parameters (2)
- BCG core radius r_c = 30 h^-1 kpc =
30 h^-1 kpc
- Scale-factor step Delta a = 0.04 =
0.04
assumptions (6)
- domain assumption Tracers (galaxies and dark matter particles) are collisionless and obey the collisionless Boltzmann equation.
- domain assumption The anisotropy of the velocity dispersion tensor is known or solved to adequate accuracy.
- domain assumption The cluster centre (most bound particle of the most massive subhalo with a 30 h^-1 kpc core average) is a valid freely falling reference frame, so the acceleration a0 in equation (3) is the gravitational acceleration of the centre.
- domain assumption The finite difference of momentum density between a = 1 and a = 1.04 accurately represents the time derivative.
- domain assumption The simulation ensemble (Millennium, Omega_m = 0.25, sigma8 = 0.9, FoF and r200 halo definition) is representative of real cluster samples for calibration.
- domain assumption Dynamics are Newtonian with the cosmological constant contributing to the acceleration.
Cite this review
Pith. "Pith review of Dynamical analysis of stacked samples of asymmetric, non-static, self-gravitating systems." pith.science (2026). https://pith.science/paper/LR227H57
@misc{pith2026250419922,
author = {Pith},
title = {Pith review of: Dynamical analysis of stacked samples of asymmetric, non-static, self-gravitating systems},
year = {2026},
howpublished = {\url{https://pith.science/paper/LR227H57}},
note = {Machine review of arXiv:2504.19922}
}
read the original abstract
We use numerical simulations to explore biases that arise in dynamical estimates of the mean mass profile for a collection of galaxy clusters that have been stacked to make a composite. There are three types of bias. One arises from anisotropy of the kinematic pressure tensor and has been already well studied; a second arises from departures from equilibrium; and a third arises because of heterogeneity of the clusters used, from their individual non-sphericity, and because velocities used are measured with respect to centres that are, in general, accelerating. Here we focus on the latter two. We stack clusters to measure the pressure tensor and density profiles and then estimate the dynamical mass profile using the Jeans equation, and compare to the actual mean mass profile. The main result of this paper is an estimate of the bias, that can be used to correct the dynamical mass estimate, and we show how it depends on the cluster sample selection. We find that Jeans equation typically overestimates the true mass by about 20\% at the virial radius.
Figures
Reference graph
Works this paper leans on
-
[1]
Alam S., Zhu H., Croft R. A. C., Ho S., Giusarma E., Schneider D. P., 2017, , 470, 2822
work page 2017
-
[2]
Amendola L., Appleby S., Avgoustidis A., Bacon D., Baker T., Baldi M., Bartolo N., Blanchard A. e. a., 2016, ArXiv e-prints
work page 2016
-
[3]
Arnold C., Puchwein E., Springel V., 2014, , 440, 833
work page 2014
-
[4]
Becker M. R., McKay T. A., Koester B., Wechsler R. H., Rozo E., Evrard A., Johnston D., Sheldon E., Annis J., Lau E., Nichol R., Miller C., 2007, , 669, 905
work page 2007
-
[5]
Biesiada M., Pi \'o rkowska A., Malec B., 2010, , 406, 1055
work page 2010
-
[6]
L., Norman M
Bryan G. L., Norman M. L., 1998, , 495, 80
1998
-
[7]
Bulbul E., Liu A., Kluge M., Zhang X., Sanders J. S., Bahar Y. E., Ghirardini V., Artis E., Seppi R., Garrel C., Ramos-Ceja M. E., Comparat J., Balzer F., B \"o ckmann K., Br \"u ggen M., Clerc N., Dennerl K., Dolag K., Freyberg M., Grandis S., Gruen D., Kleinebreil F., Krippendorf S., Lamer G., Merloni A., Migkas K., Nandra K., Pacaud F., Predehl P., Rei...
work page 2024
-
[8]
Outer regions of galaxy clusters as a new probe to test modifications to gravity
Butt M. A., Haridasu S., Diaferio A., Benetti F., Boumechta Y., Baccigalupi C., Lapi A., 2025, arXiv e-prints, p. arXiv:2504.16685
work page Pith review arXiv 2025
Show all 58 references
-
[9]
Cai Y.-C., Kaiser N., Cole S., Frenk C., 2017, , 468, 1981
2017
-
[10]
Cappellari M., 2008, , 390, 71
2008
-
[11]
G., Yee H
Carlberg R. G., Yee H. K. C., Ellingson E., 1997, , 478, 462
1997
-
[12]
G., Yee H
Carlberg R. G., Yee H. K. C., Ellingson E., Morris S. L., Abraham R., Gravel P., Pritchet C. J., Smecker-Hane T., Hartwick F. D. A., Hesser J. E., Hutchings J. B., Oke J. B., 1997a, , 485, L13
-
[13]
G., Yee H
Carlberg R. G., Yee H. K. C., Ellingson E., Morris S. L., Abraham R., Gravel P., Pritchet C. J., Smecker-Hane T., Hartwick F. D. A., Hesser J. E., Hutchings J. B., Oke J. B., 1997b, , 476, L7
-
[14]
J., Prada F., Klypin A., Moles M., 2008, , 389, 385
Cuesta A. J., Prada F., Klypin A., Moles M., 2008, , 389, 385
2008
-
[15]
S., White S
Davis M., Efstathiou G., Frenk C. S., White S. D. M., 1985, , 292, 371
1985
-
[16]
J., Andrade-Santos F., Ragone-Figueroa C., Rasia E., Forman W., Jones C., Kipper R., Borgani S., Lambas D
De Propris R., West M. J., Andrade-Santos F., Ragone-Figueroa C., Rasia E., Forman W., Jones C., Kipper R., Borgani S., Lambas D. G., Romashkova E. A., Patra K. C., 2021, , 500, 310
2021
-
[17]
E., Allende Prieto C., Annis J., Bailey S., Balland C., et al
DESI Collaboration Aghamousa A., Aguilar J., Ahlen S., Alam S., Allen L. E., Allende Prieto C., Annis J., Bailey S., Balland C., et al. 2016, ArXiv e-prints
2016
-
[18]
Falck B., Koyama K., Zhao G.-B., 2015, , 7, 049
2015
-
[19]
A., Wojtak R., Hansen S
Falco M., Mamon G. A., Wojtak R., Hansen S. H., Gottl \"o ber S., 2013, , 436, 2639
2013
-
[20]
Fong M., Han J., 2021, , 503, 4250
2021
-
[21]
I., Frenk C
Genina A., Read J. I., Frenk C. S., Cole S., Ben \' tez-Llambay A., Ludlow A. D., Navarro J. F., Oman K. A., Robertson A., 2020, , 498, 144
2020
-
[22]
M., Hivon E., Banday A
G \'o rski K. M., Hivon E., Banday A. J., Wandelt B. D., Hansen F. K., Reinecke M., Bartelmann M., 2005, , 622, 759
2005
-
[23]
F., Winther H
Gronke M., Llinares C., Mota D. F., Winther H. A., 2015, , 449, 2837
2015
-
[24]
J., Ruiz-Macias O., Cole S., Weinberg D
Hahn C., Wilson M. J., Ruiz-Macias O., Cole S., Weinberg D. H., Moustakas J., Kremin A., Tinker J. L., Smith A., Wechsler R. H., DESI collaboration 2023, , 165, 253
2023
-
[25]
A., Koester B
Hao J., McKay T. A., Koester B. P., Rykoff E. S., Rozo E., Annis J., Wechsler R. H., Evrard A., Siegel S. R., Becker M., Busha M., Gerdes D., Johnston D. E., Sheldon E., 2010, , 191, 254
2010
-
[26]
P., McCarthy I
Harvey D., Courbin F., Kneib J. P., McCarthy I. G., 2017, , 472, 1972
2017
-
[27]
Jain B., Taylor A., 2003, Physical Review Letters, 91, 141302
2003
-
[28]
Jimeno P., Broadhurst T., Coupon J., Umetsu K., Lazkoz R., 2015, , 448, 1999
2015
-
[29]
R., Postman M., Strauss M
Lauer T. R., Postman M., Strauss M. A., Graves G. J., Chisari N. E., 2014, , 797, 82
2014
-
[30]
H., Mo H
Lim S. H., Mo H. J., Lu Y., Wang H., Yang X., 2017, , 470, 2982
2017
-
[31]
Lombriser L., Koyama K., Zhao G.-B., Li B., 2012, , 85, 124054
2012
-
[32]
A., Biviano A., Bou \'e G., 2013, , 429, 3079
Mamon G. A., Biviano A., Bou \'e G., 2013, , 429, 3079
2013
-
[33]
T., Collins C
Mpetha C. T., Collins C. A., Clerc N., Finoguenov A., Peacock J. A., Comparat J., Schneider D., Capasso R., Damsted S., Furnell K., Merloni A., Padilla N. D., Saro A., 2021, , 503, 669
2021
-
[34]
F., Frenk C
Navarro J. F., Frenk C. S., White S. D. M., 1996, , 462, 563
1996
-
[35]
A., Pearce F
Old L., Skibba R. A., Pearce F. R., Croton D., Muldrew S. I., Mu \ n oz-Cuartas J. C., Gifford D., Gray M. E., von der Linden A., Mamon G. A., Merrifield M. R., M \"u ller V., Pearson R. J., Ponman T. J., Saro A., Sepp T., Sif \'o n C., Tempel E., Tundo E., Wang Y. O., Wojtak ...
2014
-
[36]
M., Dossena S., Butt M
Pizzuti L., Boumechta Y., Haridasu S., Pombo A. M., Dossena S., Butt M. A., Benetti F., Baccigalupi C., Lapi A., 2024, , 2024, 014
2024
-
[37]
I., Steger P., 2017, , 471, 4541
Read J. I., Steger P., 2017, , 471, 4541
2017
-
[38]
Richardson T., Fairbairn M., 2013, , 432, 3361
2013
-
[39]
Rosselli D., Marulli F., Veropalumbo A., Cimatti A., Moscardini L., 2023, , 669, A29
2023
-
[40]
S., Rozo E., Busha M
Rykoff E. S., Rozo E., Busha M. T., Cunha C. E., Finoguenov A., Evrard A., Hao J., Koester B. P., Leauthaud A., Nord B., Pierre M., Reddick R., Sadibekova T., Sheldon E. S., Wechsler R. H., 2014, , 785, 104
2014
-
[41]
L., Lahav O., 2015, , 114, 071103
Sadeh I., Feng L. L., Lahav O., 2015, , 114, 071103
2015
-
[42]
Schmidt F., 2010, , 81, 103002
2010
-
[43]
Shi R., Wang W., Li Z., Zhu L., Smith A., Cole S., Gao H., Chen X., Li Q., Han J., 2024, , 973, 82
2024
-
[44]
S., Greene T., Guyon O
Spergel D., Gehrels N., Breckinridge J., Donahue M., Dressler A., Gaudi B. S., Greene T., Guyon O. e. a., 2013, ArXiv e-prints
2013
-
[45]
F., Frenk C
Springel V., Wang J., Vogelsberger M., Ludlow A., Jenkins A., Helmi A., Navarro J. F., Frenk C. S., White S. D. M., 2008, , 391, 1685
2008
-
[46]
Springel V., White S. D. M., Jenkins A., Frenk C. S., Yoshida N., Gao L., Navarro J., Thacker R., Croton D., Helly J., Peacock J. A., Cole S., Thomas P., Couchman H., Evrard A., Colberg J., Pearce F., 2005, , 435, 629
2005
-
[47]
J., 2012, aap, 540, A106
Tempel E., Tago E., Liivam \"a gi L. J., 2012, aap, 540, A106
2012
-
[48]
P., Magorrian J., Carlberg R
van der Marel R. P., Magorrian J., Carlberg R. G., Yee H. K. C., Ellingson E., 2000, , 119, 2038
2000
-
[49]
L., van de Ven G., den Brok M., van den Bosch R
Watkins L. L., van de Ven G., den Brok M., van den Bosch R. C. E., 2013, , 436, 2598
2013
-
[50]
L., Han J
Wen Z. L., Han J. L., 2024, , 272, 39
2024
-
[51]
L., Han J
Wen Z. L., Han J. L., Liu F. S., 2012, , 199, 34
2012
-
[52]
H., Hjorth J., 2011, , 477, 567
Wojtak R., Hansen S. H., Hjorth J., 2011, , 477, 567
2011
-
[53]
J., van den Bosch F
Yang X., Mo H. J., van den Bosch F. C., Pasquali A., Li C., Barden M., 2007, , 671, 153
2007
-
[54]
Yang X., Xu H., He M., Gu Y., Katsianis A., Meng J., Shi F., Zou H., Zhang Y., Liu C., Wang Z., Dong F., Lu Y., Li Q., Chen Y., Wang H., Mo H., Fu J., Guo H., Leauthaud A., Luo Y., Zhang J., Zu Y., 2021, , 909, 143
2021
-
[55]
Zhang P., Liguori M., Bean R., Dodelson S., 2007, Physical Review Letters, 99, 141302
2007
-
[56]
Zhao G.-B., Li B., Koyama K., 2011, Physical Review Letters, 107, 071303
2011
-
[57]
H., Jennings E., Li B., Wyman M., 2014, , 445, 1885
Zu Y., Weinberg D. H., Jennings E., Li B., Wyman M., 2014, , 445, 1885
2014
-
[58]
write newline
" write newline "" before.all 'output.state := FUNCTION fin.entry write newline FUNCTION new.block output.state before.all = 'skip after.block 'output.state := if FUNCTION new.sentence output.state after.block = 'skip output.state before.all = 'skip after.sentence 'output.stat...
Reviewed August 16, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.