MOCCA survey database I: preliminary mock Extra Galactic Globular Cluster observations

Agostino Leveque; Miros{\l}aw Giersz

REVIEW 3 major objections 5 minor 11 references

MOCCA survey database I: preliminary mock Extra Galactic Globular Cluster observations

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Simulated 12 Gyr old globular clusters with different internal dynamical states—early IMBH, late IMBH, black-hole subsystem, or standard—separate in integrated $V-I$ color when viewed as unresolved extragalactic objects.

desk verdict Interesting idea, weak evidence: the claimed dynamical-state signature in V-I color is confounded by uncontrolled initial conditions and non-representative model selection. read the letter →

arxiv 1908.02033 v1 pith:V757RICX submitted 2019-08-06 astro-ph.GA

Agostino Leveque , Miros{l}aw Giersz This is my paper · ORCID

classification astro-ph.GA

keywords globularclustersextragalacticstellarpopulationsynthesismockobservationsintermediate-massblackholesholesubsystemscolorbimodalityMOCCAsurvey

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper asks whether the internal dynamical state of a globular cluster leaves a measurable trace in the integrated light observed for unresolved extragalactic clusters. Using 12 Gyr old simulations from the MOCCA Survey Database, the authors place clusters in four dynamical-state groups—hosting an intermediate-mass black hole formed before 1 Gyr, after 1 Gyr, hosting a black-hole subsystem with more than 75 black holes, or neither—and synthesize mock observations in the U, B, V, R, and I bands. They report that the four groups separate in $V-I$ color, with a spread of about 0.2 and a distance-dependent shift of about 0.5 at 17 Mpc, while the same clusters at 9 Gyr are redder and less spread out. The paper is a proof of concept: if the correlation holds in a larger, better-controlled sample, integrated color could serve as a cheap observational probe of a globular cluster's dynamical history, including the presence of an IMBH or black-hole subsystem.

What carries the argument

The working machinery is a two-stage bridge from dynamics to observables. First, the MOCCA Survey Database provides 12 Gyr old cluster snapshots with known dynamical states, classified by whether an IMBH formed early or late, whether a black-hole subsystem with more than 75 black holes is present, or neither. Second, a synthetic-observation pipeline converts those snapshots into mock extragalactic images: the FSPS stellar population synthesis code assigns each star absolute magnitudes in U, B, V, R, and I from its radius, mass, and luminosity, and the COCOA imaging code places the projected cluster at nine distances from 1 to 17 Mpc using camera parameters of 0.025 arcsec per pixel, 0.8 arcsec seeing, and no noise or background. The integrated $V-I$ color and half-light radius are then measured from the mock surface brightness, turning an internal dynamical-state label into an externally visible photometric property.

What would settle it

Select a matched subsample of MOCCA models with equal numbers per dynamical-state group and identical distributions of initial mass, half-mass radius, and orbital parameters, then redo the mock-observation color comparison at 12 Gyr. If the $V-I$ offsets among the four groups disappear or fall below the within-group scatter, the reported dynamical-state color signal is an artifact of uncontrolled initial conditions; if they persist, the color–dynamical-state link is real.

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Extended reading notes

Core claim

The authors' central claim is that the integrated $V-I$ color of a 12 Gyr old globular cluster, as it would be measured for an unresolved source at distances from 1 to 17 Mpc, depends on the cluster's internal dynamical state. Clusters classified as FIMBH (an intermediate-mass black hole with mass above $150\,M_\odot$ formed before 1 Gyr), SIMBH (the same formation after 1 Gyr), BHS (a black-hole subsystem with more than 75 black holes), or Standard (neither) occupy distinguishable positions in color, and the authors write that 'the color differs depending on the cluster structure.' They also find that the color is a strong function of cluster age: at 9 Gyr the same groups show less color broadening and a higher typical $V-I$ value than at 12 Gyr, which they attribute to stellar evolution. The paper explicitly frames this as a preliminary result and notes that initial cluster parameters also influence the color, so the dynamical-state signal sits on top of an uncontrolled initial-condition spread.

Load-bearing premise

The load-bearing assumption is that the $V-I$ color offsets between clusters with an early-formed IMBH, a late-formed IMBH, a black-hole subsystem, or neither come from that dynamical state itself, even though the models in each group were not matched on initial mass, structure, or orbit—so the offsets could reflect correlated initial conditions instead.

Editorial extensions

If this is right

If the color–dynamical-state separation holds, integrated $V-I$ photometry of unresolved extragalactic globular clusters can flag candidates for IMBH-hosting or black-hole-dominated clusters without resolving individual stars.
Because all models share metallicity $Z=0.001$, real clusters with different metallicities will show a broader $V-I$ distribution, so metallicity must be measured or marginalized before the dynamical-state signal can be extracted from observed color histograms.
The strong age dependence—9 Gyr models are redder and less spread than 12 Gyr models—means age must be known or controlled before using color as a dynamical-state indicator.
The distance and redshift effect shifts $V-I$ by about 0.5 at the largest modeled distance (17 Mpc), so distance corrections are necessary when comparing clusters across a galaxy or between galaxies.
A straightforward next step within the paper's own method is to apply the same mock-observation pipeline to the full roughly 2000-model survey, which would test whether the four-group color offsets survive sample-size and initial-condition variations.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The roughly 0.2 spread in simulated $V-I$ color is the same order of magnitude as the observed blue-to-red peak separation in bright early-type galaxies, so internal dynamical-state variation is a plausible contributor to scatter within each of the observed metallicity peaks—an extension the paper leaves implicit.
If the color offsets are really dynamical in origin, the method could be inverted: one could fit observed extragalactic globular cluster color distributions with mixtures of FIMBH, SIMBH, BHS, and Standard templates to estimate the fraction of clusters in each dynamical state in a galaxy.
A matched-initial-condition rerun, if it eliminates the offsets, would not destroy the method but would redirect it: the colors would then trace the initial conditions (mass, concentration, orbit) rather than the subsequent dynamical pathway, which is itself a useful observable.
The redshift shift of about 0.5 in $V-I$ at 17 Mpc is large enough that k-corrected colors will be essential; the paper's no-noise, no-background setup can be extended to realistic noise to test whether the group separation survives real observational conditions.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The core idea is appealing: if integrated V-I colors of extragalactic globular clusters could signal the presence of an intermediate-mass black hole or a black-hole subsystem, that would give a cheap probe of internal dynamics where we can't resolve stars. The paper takes 47 MOCCA simulated clusters at fixed metallicity and age, synthesizes their photometry with FSPS, makes mock LBT images with COCOA, and compares V-I colors across four dynamical-state groups (FIMBH, SIMBH, BHS, Standard). That's a legitimate new application of established tools, and the authors are upfront that this is a proof of concept.

The problem is that the central claim in Section 4—that color 'clearly differs depending on the cluster structure'—is not actually supported by the analysis. The groups are explicitly drawn from models with unconstrained initial conditions, and IMBH formation correlates with initial mass and concentration, which also shape the stellar population and integrated light. The authors concede this, but then still treat the group differences as a dynamical-state signature. With 8 to 15 models per group, no matched initial-condition control, and no significance tests, the between-group color differences could easily be a selection artifact. Figure 1 makes it worse by picking, for each group, the model with the largest color spread; that's cherry-picking the extremes, not illustrating a typical trend. Also, the abstract promises half-light radii and color spatial distribution, but the results only show colors; the half-light radii analysis is missing.

There are minor issues too, like a few typos, a vague description of how the Fig. 1 models were selected, and no code or data release. But the main weakness is the uncontrolled comparison.

What works: the pipeline is sensible, FSPS and COCOA are independent standard packages, and this appears to be the first comparison of synthetic EGGC colors across MOCCA dynamical classes. The qualitative idea is worth testing, just not with this analysis alone.

For an IAU proceedings paper, this is acceptable as a preliminary pilot if the claim is softened to a 'promising trend needing controlled follow-up.' For a full journal paper, I'd ask for matching initial conditions, statistical tests, and the promised half-light radii. It deserves a serious referee in the sense that a thoughtful reviewer would catch exactly these issues and help the authors design a cleaner study. I wouldn't cite it yet, but it's a reasonable pointer to an interesting direction.

Referee Report

3 major / 5 minor

Summary. The paper presents a proof-of-concept study in which 12 Gyr old globular cluster models from the MOCCA Survey Database are divided into four dynamical-state groups (FIMBH, SIMBH, BHS, Standard) and converted into mock extragalactic observations using FSPS stellar photometry and the COCOA image tool. Integrated V-I colors are computed for clusters placed at distances from 1 to 17 Mpc, and the paper claims that the colors differ according to the dynamical state of the cluster. The abstract also promises measurements of half-light radii, but no such results appear in the text.

Significance. If the claimed color differences were robust, this would be an interesting proof of concept that integrated extragalactic globular cluster colors might encode information about the internal dynamical state, not only metallicity and age. The pipeline has genuine strengths: it uses independently developed external tools (FSPS and COCOA), the synthesis step is not fitted to the photometric output, and the approach is falsifiable in the sense that it makes a concrete prediction for mock observations. However, the sample is small, the groups are not matched in initial conditions, the illustrative model selection is biased toward large color spreads, and no statistical test is provided; the evidence as presented supports only a tentative, hypothesis-generating statement, not the strong conclusion stated in Section 4.

major comments (3)

[§4, Figs. 1–2] The central claim that V−I color "clearly ... differs depending on the cluster structure" is not established because the four dynamical-state groups are not matched in initial conditions. Section 2 states "We do not impose any constrains on the initial conditions", and Section 4 concedes "the initial GC parameters also have some influence on the color". Since formation of an IMBH or a black-hole subsystem depends on initial cluster mass and concentration, the between-group color differences could be caused by correlated initial conditions rather than by the dynamical state itself. The authors need a controlled comparison (e.g., sub-samples matched in initial mass, half-mass radius, and concentration) or a regression including those initial parameters as covariates.
[§4, Fig. 1] The four displayed models are chosen as the models with the largest V−I color spread in each dynamical-state group. This selection procedure maximizes the apparent group separation and makes Figure 1 unsuitable as evidence for a dynamical-state signature. The claim should be based on the full sample in Figure 2, supplemented by a quantitative measure of the separation between group means relative to the within-group scatter, rather than on hand-picked extrema.
[§4, Figs. 1–3] No significance test or uncertainty quantification is given for the between-group color differences. The standard deviations in Figure 2 appear to be of the same order as the claimed color spread, so the statement "we can clearly see how the color differs" is not quantitatively supported. A bootstrap or permutation test on the group means, or an ANOVA-type decomposition of between-group versus within-group variance, is needed because this comparison is the sole basis for the paper's conclusion.

minor comments (5)

[Abstract] The abstract states that half-light radii are measured from the surface brightness, but no half-light radius results are reported anywhere in the paper; the authors should either present these results or remove the promise from the abstract.
[§2] There are several typographical errors: "constrains" should be "constraints", "properities" should be "properties", and "variuos" should be "various".
[§4] The text says the color change is "~0.5 dex at the largest distance", but V−I color is a magnitude difference and is normally quoted in magnitudes, not dex; please correct the unit and, if the change is actually 0.05 mag, fix the value.
[Fig. 2 caption] The caption says the bar shows the standard deviation "for each model", but the text describes the points as the mean color over the sample; please clarify whether the error bars represent model-to-model scatter in the group or uncertainty on the mean.
[§3] The distance grid and redshift effect are mentioned, but the assumed redshifts for the nine distances are not given; without this, the near-independence of color with distance in Figure 1 cannot be assessed.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: mock photometry pipeline is independent of the color claim.

full rationale

The derivation chain is not circular. The four dynamical-state groups (FIMBH, SIMBH, BHS, Standard) are defined by simulated dynamical properties — IMBH formation time or presence of a black-hole subsystem — not by V-I color. Colors are computed by applying the external FSPS and COCOA packages to projected simulation snapshots; no parameter is fitted to the target V-I colors, and the color output is not fed back into the group definitions. The MOCCA database and COCOA are prior work by the same group, but they are used as input simulation products and do not presuppose a relation between color and dynamical state. The paper itself flags the main non-circular caveat: 'We do not impose any constrains on the initial conditions' and 'the initial GC parameters also have some influence on the color,' so between-group color differences may be confounded by correlated initial conditions. That is a correctness/interpretation limitation, not a circular reduction. The Fig. 1 selection of 'models with the largest spread in V−I color' is an illustrative-selection caveat, but the overall comparison is summarized in Fig. 2, and the central claim is not forced by construction. No circular step is therefore identified.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The paper fits no parameters to data; it forwards MOCCA model outputs through FSPS and COCOA with fixed inputs, including metallicity Z=0.001, ages 9 and 12 Gyr, nine distances, and LBC-like instrument settings. The principal ledger entries are domain assumptions about the fidelity of the MOCCA database, the FSPS stellar-photometry modification, and the representativeness of extreme-model selection.

assumptions (4)

domain assumption MOCCA survey models are representative of the Milky Way globular cluster population.
Section 2 invokes Askar et al. (2017); if the initial conditions and dynamical evolution encoded in the database are not representative, the synthetic EGGC colors will not transfer to real clusters.
domain assumption The modified FSPS code gives accurate broadband magnitudes for individual stars from input mass, radius, and luminosity.
Section 3 footnote describes modifying FSPS for individual stellar photometry but gives no validation in this paper; all derived colors depend on this conversion.
ad hoc to paper The four displayed models, chosen for the largest V-I color spread in their class, are informative representatives of their dynamical state groups.
Figure 1 caption states the selection rule; without a random or matched sample the displayed differences are extreme examples, not group statistics.
domain assumption The dynamical-state classification thresholds (IMBH formed before or after 1 Gyr with mass greater than 150 solar masses, BH subsystem with more than 75 black holes) define observationally meaningful classes.
Section 2 defines the four groups using these thresholds from prior MOCCA work; the color comparison assumes these classes, not the thresholds themselves, drive any photometric difference.

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Cite this review

Pith. "Pith review of MOCCA survey database I: preliminary mock Extra Galactic Globular Cluster observations." pith.science (2026). https://pith.science/paper/V757RICX

@misc{pith2026190802033,
  author       = {Pith},
  title        = {Pith review of: MOCCA survey database I: preliminary mock Extra Galactic Globular Cluster observations},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/V757RICX}},
  note         = {Machine review of arXiv:1908.02033}
}

read the original abstract

The photometric properties that we could observe for Extra-Galactic Globular Clusters (EGGCs) are the integrated light of the system and for nearby EGGCs it also is possible to measure both half-light radii and the color spatial distribution, e.g. for areas smaller and larger than the half-light radius. No information about the internal dynamical state of the system could be directly obtained from observations. On the other hand, simulations of Globular Clusters (GCs) can provide detailed information about the dynamical evolution of the system. We present a preliminary study of EGGCs' photometric properties for different dynamical evolutionary stages. We apply this study to 12 Gyr old GCs simulated as part of the MOCCA Survey Database. We determine the magnitudes in different bands from their projected snapshots using the Flexible Stellar Population Synthesis (FSPS) code and we measure the half-light radii from the surface brightness.

Figures

Figures reproduced from arXiv: 1908.02033 by the authors.

**Figure 1.** The computed V − I color as a function of the GC distance for chosen models. representative of the Milky Way GC population. More details are given in Askar et al. (2017). We selected models according to their dynamical state at 12 Gyr, dividing them into four groups: • Fast scenario (FIMBH), presence of an IMBH (BH with masses > 150M ) formed before 1 Gyr (Giersz et al. 2015); • Slow scenario (SIMBH), presence of an… view at source ↗

**Figure 2.** The mean V − I color over our sample as a function of the GC distance. The bar shows the standard deviation for each model. 4. Results Results at 12 Gyr In [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** The computed V − I color at 9 Gyr as a function of the GC distance for chosen models. Acknowledgements MG and AL were partially supported by the Polish National Science Center (NCN) through the grant UMO-2016/23/B/ST9/02732. References Ashman, K. M & Zepf, S. E. 1992, ApJ, 384, 50 Forbes, D. A., Brodie, J. P. & Grillmair, C. J. 1997a, AJ, 113, 1652 Cˆot´e, P., Marzke, R. O. & West, M. J. 1998, ApJ, 501, 554 Brodie, … view at source ↗

Discussion (0). Continue with ORCID to comment.

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

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