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arxiv: 1906.11863 · v1 · pith:4Y6YMVWEnew · submitted 2019-06-27 · 🌌 astro-ph.GA · astro-ph.SR

Another relic bulge globular cluster: ESO 456-SC38 (Djorgovski~2)

S. Ortolani , E.V. Held , D. Nardiello , S. Souza , B. Barbuy , A. P\'erez-Villegas , S. Cassisi , E. Bica
show 2 more authors
Y. Momany I. Saviane
This is my paper

Pith reviewed 2026-05-25 14:19 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.SR
keywords globular clustersMilky Way bulgeisochrone fittingstellar populationsGalactic structureDjorgovski 2ESO 456-SC38
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The pith

ESO 456-SC38 is an old globular cluster at 8.75 kpc in the inner Galactic bulge with metallicity -1.11 and age 12.7 Gyr.

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

The paper determines precise distance, reddening, age and metallicity for the faint, field-contaminated globular cluster ESO 456-SC38 using combined HST, VLT and Gaia photometry. Markov-chain Monte Carlo fitting of BaSTI isochrones yields an age of 12.7 Gyr, [Fe/H] = -1.11 and placement on the blue horizontal branch. These parameters align the cluster with a small group of very old, moderately metal-poor clusters already known in the inner bulge. The cluster remains confined to the bulge and bar region yet shows no structural support for the Galactic bar itself. The findings require bulge-formation models to incorporate such ancient stellar populations when tracing the Milky Way's earliest assembly.

Core claim

ESO 456-SC38 lies at dSun = 8.75 kpc with E(B-V) = 0.81, has age 12.70 Gyr and [Fe/H] = -1.11, sits on the blue horizontal branch, and belongs to the inner bulge without supporting the bar structure, thereby extending the known sample of primeval inner-bulge globular clusters.

What carries the argument

MCMC fitting of BaSTI isochrones to multi-band HST, VLT and Gaia photometry that simultaneously solves for distance, reddening, age and metallicity while accounting for field contamination.

If this is right

  • ESO 456-SC38 joins the small set of moderately metal-poor, blue-horizontal-branch clusters already mapped in the inner bulge.
  • The old stellar population traced by such clusters must be included in any model of Galactic bulge formation.
  • The cluster supplies an additional datum on the formation epoch of globular clusters themselves.
  • The object remains dynamically confined to the bulge-bar volume but does not trace the bar's elongated structure.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Orbital integration of this cluster together with the other inner-bulge objects could reveal whether they share a common dynamical history.
  • High-resolution spectroscopy of member stars would test whether the chemical abundance patterns match those of the other ancient bulge clusters.
  • Deeper photometry reaching the main-sequence turn-off in less contaminated fields could tighten the age uncertainty.

Load-bearing premise

The BaSTI isochrone models correctly describe the color-magnitude diagram of this cluster once field stars and differential reddening are removed.

What would settle it

A spectroscopic metallicity measurement or independent age indicator that lies well outside [Fe/H] = -1.11 or 12.7 Gyr would falsify the isochrone solution.

Figures

Figures reproduced from arXiv: 1906.11863 by A. P\'erez-Villegas, B. Barbuy, D. Nardiello, E. Bica, E.V. Held, I. Saviane, S. Cassisi, S. Ortolani, S. Souza, Y. Momany.

Figure 2
Figure 2. Figure 2: FORS2 V image of ESO456-SC38. North is up, and east to the left. The field size is 7×7 arcmin2 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Color-magnitude diagram mF606W vs. mF606W -mF110W (left panel) of ESO456-SC38 from HST/ACS and WFC3 observa￾tions fit with the statistical Bayesian MCMC method (SIRIUS code), based on a grid of BaSTI isochrones. The green solid lines are the most probable solution, and the green region represents the solutions inside 1σ from the posterior distributions. Large red dots are stars identified in Gaia within 25… view at source ↗
Figure 4
Figure 4. Figure 4: Corner plot relative to the mF606W vs. mF606W -mF110W CMD in [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: I vs. V − I CMD from FORS2, overimposed with a BaSTi isochrone of 12.7 Gyr and [Fe/H]=−1.1 (Z=0.004) (ma￾genta line). Stars within 0.9 arcmin of cluster center are se￾lected. The mean magnitudes and colors of 7 RR Lyrae are shown with blue crosses. Black dots are stars within 18 arc￾sec from the cluster center. Large red dots are stars identified in Gaia within 25 arcsec of cluster center. 1997) to model t… view at source ↗
Figure 6
Figure 6. Figure 6: Distribution of Gaia-identified stars in proper motion space, with errors given in Gaia DR2 [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Probability density map for the x − y and R − z projections of the 1000 orbits for ESO456-SC38. The orbits corotate with the bar frame. The red and yellow colors correspond to the higher probabilities. The black lines show the orbits using the central values presented in [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Distribution of orbital parameters for ESO456-SC38, perigalactic distance rmin, apogalactic distance rmax, maximum vertical excursion from the Galactic plane |z|max, and eccentricity. The colors show the different angular speed of the bar, Ωb = 40 (blue), 45 (orange), and 50 (green) km s−1 kpc−1 . The source ESO456-SC38 is an addition to the list of mod￾erately metal-poor ([Fe/H]∼ −1.0) clusters on the blu… view at source ↗
Figure 9
Figure 9. Figure 9: Same as [PITH_FULL_IMAGE:figures/full_fig_p008_9.png] view at source ↗
read the original abstract

The object ESO456-SC38 (Djorgovski 2) is one of the globular clusters that is closest to the Galactic center. It is on the blue horizontal branch and has a moderate metallicity of [Fe/H]~-1.0. It is thus similar to the very old inner bulge globular clusters NGC 6522, NGC 6558, and HP 1, and therefore appears to be part of the primeval formation stages of the Milky Way. The aim of this work is to determine an accurate distance and metallicity for ESO456-SC38, as well as orbital parameters, in order to check similarities with other clusters in the inner bulge that have previously been well studied in terms of spectroscopy and photometry. This is a considerably fainter cluster that is contaminated by a rich stellar field; it is also quite absorbed by the dusty foreground. We analyzed ESO456-SC38 based on Hubble Space Telescope photometry, with the filters F606W from ACS, F110W and F160W from WFC3,and photometry in V and I from FORS2 at the VLT. We combined this with identified stars that are covered by Gaia Data Release 2. The isochrone fitting was carried out with the statistical Markov chain Monte Carlo method. We derive an accurate distance of dSun = 8.75+-0.12 kpc and a reddening of E(B-V)=0.81^+0.02_-0.02. The best-fitting BaSTI isochrones correspond to an age of 12.70^{+0.72}_{-0.69} Gyr and a metallicity of [Fe/H]=-1.11^{+0.03}_{-0.03}. ESO 456-SC38 adds to the list of moderately metal-poor globular clusters located in the inner bulge. It is on the blue horizontal branch and is very old. The cluster is confined to the bulge and bar region, but it does not support the Galactic bar structure. The old stellar population represented by clusters like this has to be taken into account in models of Galactic bulge formation. Studying them also provides indications on the formation times of the globular clusters themselves.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper analyzes ESO 456-SC38 (Djorgovski 2) using combined HST ACS/WFC3 (F606W, F110W, F160W), VLT FORS2 (V, I), and Gaia DR2 photometry of this faint, field-contaminated, and reddened inner-bulge globular cluster. MCMC isochrone fitting with BaSTI models yields d_Sun = 8.75 ± 0.12 kpc, E(B-V) = 0.81, age = 12.70 Gyr, [Fe/H] = -1.11, blue horizontal-branch morphology, and orbital parameters indicating confinement to the bulge/bar region without supporting the bar structure, positioning the cluster as a relic similar to NGC 6522, NGC 6558, and HP 1.

Significance. If the fitted parameters hold, the work enlarges the sample of well-characterized old, moderately metal-poor inner-bulge clusters, strengthening evidence for an early-formed stellar population in the Galactic bulge and the requirement to include such objects in bulge-formation models. The multi-filter photometry and statistical MCMC approach provide a reproducible route to the quoted posteriors.

major comments (3)
  1. [isochrone fitting section] Isochrone fitting section: The MCMC procedure fits the four free parameters (age, [Fe/H], distance modulus, E(B-V)) to the combined photometry, but the manuscript provides no explicit description of the field-contamination model (e.g., the adopted luminosity function or membership probability weighting) or the differential-reddening map used in the likelihood; given the abstract's emphasis on heavy contamination and absorption, residual mismatches directly shift the reported posteriors on age, metallicity, and distance.
  2. [results and discussion sections] Results and discussion sections: The classification as a 'relic bulge globular cluster' and the statement that it 'does not support the Galactic bar structure' rest entirely on the MCMC-derived parameters and subsequent orbital integration; no sensitivity tests to alternative isochrone libraries (e.g., PARSEC) or independent spectroscopic [Fe/H] anchors are reported, leaving the central claim vulnerable to systematic offsets in the BaSTI models at [Fe/H] ≈ -1.1.
  3. [isochrone fitting section] Abstract and isochrone fitting section: The quoted uncertainties (e.g., age 12.70^{+0.72}_{-0.69} Gyr) are presented as statistical only; the text does not propagate or discuss systematic uncertainties arising from the choice of isochrone set, reddening law, or contamination treatment, which are load-bearing for the comparison to NGC 6522/6558/HP 1.
minor comments (2)
  1. [abstract] The abstract states [Fe/H] ~ -1.0 while the fit reports -1.11; a single consistent value or explicit rounding statement would improve clarity.
  2. Figure captions and text should explicitly state the number of stars retained after Gaia proper-motion selection and the magnitude range used in the MCMC fit.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the thorough and constructive report. We address each of the major comments below and indicate the revisions we will make to the manuscript.

read point-by-point responses

  1. Referee: [isochrone fitting section] Isochrone fitting section: The MCMC procedure fits the four free parameters (age, [Fe/H], distance modulus, E(B-V)) to the combined photometry, but the manuscript provides no explicit description of the field-contamination model (e.g., the adopted luminosity function or membership probability weighting) or the differential-reddening map used in the likelihood; given the abstract's emphasis on heavy contamination and absorption, residual mismatches directly shift the reported posteriors on age, metallicity, and distance.

    Authors: We agree with the referee that the description of the contamination model and differential reddening was not sufficiently explicit. The revised manuscript will include a new subsection detailing the field star luminosity function used, the membership probability weighting in the likelihood, and how the differential reddening map was constructed and applied. This will allow full reproduction of the fitting procedure. revision: yes

  2. Referee: [results and discussion sections] Results and discussion sections: The classification as a 'relic bulge globular cluster' and the statement that it 'does not support the Galactic bar structure' rest entirely on the MCMC-derived parameters and subsequent orbital integration; no sensitivity tests to alternative isochrone libraries (e.g., PARSEC) or independent spectroscopic [Fe/H] anchors are reported, leaving the central claim vulnerable to systematic offsets in the BaSTI models at [Fe/H] ≈ -1.1.

    Authors: The central claims are based on the parameters derived from the BaSTI isochrones and the orbital integration using those parameters. While we did not perform sensitivity tests with other libraries such as PARSEC, the BaSTI models are well-suited for this metallicity and age range. No spectroscopic [Fe/H] measurements exist for this cluster, so we rely on the photometric derivation. In the revision, we will add a discussion of possible systematic effects from isochrone choice and include a note on the absence of spectroscopic data. We will also test the orbital parameters with slightly varied inputs to assess robustness of the 'does not support the bar' conclusion. revision: partial

  3. Referee: [isochrone fitting section] Abstract and isochrone fitting section: The quoted uncertainties (e.g., age 12.70^{+0.72}_{-0.69} Gyr) are presented as statistical only; the text does not propagate or discuss systematic uncertainties arising from the choice of isochrone set, reddening law, or contamination treatment, which are load-bearing for the comparison to NGC 6522/6558/HP 1.

    Authors: We concur that systematic uncertainties were not discussed. The revised manuscript will include an expanded section on error analysis that estimates the impact of isochrone library choice, reddening law variations, and contamination treatment on the derived parameters. This will be used to qualify the comparisons with the other inner bulge clusters. revision: yes

Circularity Check

0 steps flagged

No circularity: parameters derived from external isochrone library fit to public photometry

full rationale

The paper's derivation consists of MCMC isochrone fitting using the external BaSTI models to combined HST ACS/WFC3, VLT FORS2, and Gaia DR2 photometry of ESO456-SC38. All reported values (dSun = 8.75 kpc, E(B-V) = 0.81, age 12.70 Gyr, [Fe/H] = -1.11) are direct posterior outputs of this fit to independent observational data; the relic-bulge classification follows from comparing these fitted numbers to literature values for other clusters. No equation or step reduces a claimed prediction to a quantity defined by the fit itself, no self-citation supplies a load-bearing uniqueness theorem or ansatz, and the method does not rename a known result. The derivation is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

4 free parameters · 2 axioms · 0 invented entities

The central claim rests on the accuracy of BaSTI stellar evolution models for [Fe/H]~-1 and the assumption of a single stellar population with uniform reddening after statistical decontamination.

free parameters (4)
  • age
    Fitted via MCMC to match observed CMD
  • [Fe/H]
    Fitted via MCMC to match observed CMD
  • distance modulus
    Fitted via MCMC to match observed CMD
  • reddening E(B-V)
    Fitted via MCMC to match observed CMD
axioms (2)
  • domain assumption BaSTI isochrones provide an accurate representation of stellar evolution at [Fe/H]~-1.1 for this cluster
    Invoked for the isochrone fitting procedure
  • domain assumption The cluster is a single-age, single-metallicity population with negligible multiple populations or differential reddening effects after correction
    Required for the statistical fitting to converge on unique parameters

pith-pipeline@v0.9.0 · 5985 in / 1400 out tokens · 26060 ms · 2026-05-25T14:19:10.525756+00:00 · methodology

discussion (0)

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