REVIEW 5 major objections 4 minor 45 references
Dependence of Multi-band Absolute Magnitudes and Color Indexes of the Tip of Red Giant Branch Stars on Metallicity in the Galactic Globular Clusters
T0 review · 5 major / 4 minor · reviewed 2026-08-09 · deepseek-v4-flash
Pith's one-line read The I-band tip of the red giant branch is a constant candle only below [Fe/H] = -1.2; above it, the tip fades with metallicity, pushing the TRGB-calibrated Hubble constant to 70.86 ± 1.2 ± 0.9 km/s/Mpc.
desk verdict Low-metallicity M_I is solid, but the high-metallicity trend is mostly dust, not metallicity, and the abstract gets the sign wrong. 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 central object is the tip of the red giant branch (TRGB), the point of maximum luminosity reached by low-mass stars before the helium flash. The key identification is geometric rather than statistical: the TRGB is selected as the reddest star on the red giant branch in the Gaia color-magnitude diagram, justified by the argument that higher opacity in metal-rich stars shifts radiation to longer wavelengths and makes the tip appear as the reddest, not the brightest, point of the branch. This selection feeds magnitude measurements in seven bands, which are then fitted against cluster metallicity with exponential and linear functions to produce relations such as $M_I = 10.47 \exp(3.92[\mathrm{Fe/H}]) - 4.017$.
What would settle it
A direct spectroscopic metallicity measurement of the individual TRGB stars in a dozen metal-rich clusters, together with a check of whether the reddest CMD star is the same star that shows the helium-flash luminosity discontinuity, would settle whether the steep drop in $M_I$ above [Fe/H] > -1.2 is real. Specifically, if the reddest star in clusters like NGC 6838 turns out to be an AGB star or a large-amplitude variable, the claimed metallicity dependence would weaken.
Extended reading notes
Core claim
The central discovery is that the metallicity independence of the TRGB standard candle holds only in the metal-poor regime. Using the reddest star of the red giant branch in the Gaia $G_{\rm BP}-G_{\rm RP}$ versus $G_{\rm RP}$ diagram as the TRGB, the authors calibrate absolute magnitudes in the $V$, $I$, $G_{\rm BP}$, $G_{\rm RP}$, $J$, $H$, and $K_{\rm S}$ bands for 33 Galactic globular clusters. They find $M_I$ is flat at $-4.017$ mag for [Fe/H] below $-1.2$ and fades with increasing metallicity above it, while the near-infrared magnitudes grow brighter with metallicity and the optical bands grow fainter. Applying the updated $M_I$ to Type Ia supernova calibrations yields $H_0 = 70.86 \pm 1.2 \pm 0.9$ km s$^{-1}$ Mpc$^{-1}$, and the paper argues that metal-rich galaxies such as the LMC require a metallicity correction that earlier TRGB work omitted.
Load-bearing premise
The reddest star on the red giant branch is the true TRGB, and in metal-rich clusters this reddest point is not contaminated by dusty AGB stars, foreground stars, or variable stars.
Editorial extensions
If this is right
- TRGB distances to galaxies with [Fe/H] > -1.2 need a metallicity correction; ignoring it biases distances because the tip is fainter than assumed.
- The I-band constancy that makes TRGB a standard candle is restricted to [Fe/H] < -1.2; the corresponding Hubble constant from SNe Ia calibrated with this $M_I$ is $70.86 \pm 1.2 \pm 0.9$ km/s/Mpc.
- In near-infrared bands the metallicity trend reverses, so $J$ and $K_{\rm S}$ TRGB magnitudes are brighter for metal-rich clusters, consistent with stellar model predictions.
- Color indexes such as $(G_{\rm BP}-G_{\rm RP})$, $(V-I)$, and $(J-K_{\rm S})$ show lower dispersion with metallicity than the absolute magnitudes, offering alternative calibrators.
Reading between the lines
- If the reddest-star criterion is biased in metal-rich clusters (for example by picking circumstellar dust-enshrouded stars), the steep fading above [Fe/H] > -1.2 may be partly an extinction effect rather than a true luminosity-metallicity relation; the paper's own detection of infrared excess in two metal-rich TRGBs hints at this.
- The claimed $H_0$ shift of about 1 to 2 km/s/Mpc depends on assuming that TRGB stars in the LMC and similar galaxies have the disk metallicity rather than the halo metallicity; a direct metallicity measurement of the actual TRGB stars used in the SNe Ia calibration would test this.
- The color-metallicity relations with smaller scatter could serve as a metallicity indicator for partially resolved stellar populations, if calibrated on additional clusters.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper identifies TRGB stars in 43 (later 33) Galactic globular clusters as the reddest stars in Gaia GBP−GRP versus GRP color-magnitude diagrams, computes absolute magnitudes in the GBP, GRP, V, I, J, H, and KS bands, and fits their dependence on metallicity. The central claim is that MI is nearly constant at −4.017 ± 0.036 ± 0.027 mag for [Fe/H] < −1.2 while becoming fainter for [Fe/H] > −1.2; this calibration is then transformed into H0 = 70.86 ± 1.2 ± 0.9 km/s/Mpc via the Riess et al. (2016) formula. The paper also presents color–metallicity relations and compares the KS-band behavior with stellar models.
Significance. If the low-metallicity MI value holds, it provides a useful TRGB zero point consistent with several independent calibrations and supports the use of TRGB as a standard candle. The claimed high-metallicity trend, if intrinsic, would require corrections to TRGB distances of metal-rich galaxies and would shift H0 by 1–2 km/s/Mpc. However, the high-metallicity portion rests on only six clusters, is partly attributed by the authors themselves to circumstellar dust, and is sensitive to the post-hoc rejection of 10 of 43 candidates. The paper is therefore valuable mainly for the metal-poor calibration and for the multi-band relations, while the headline metallicity correction for metal-rich systems is not yet established.
major comments (5)
- [Section 3.6, Eq. (4)] The paper states that 'the fainter TRGB magnitude in the metal-rich stars is most likely caused by circumstellar dust' and that excluding the two TRGBs with infrared excess 'shows that the trend ... slows down.' Yet Eq. (4) and the H0 application use the full fit as an intrinsic metallicity dependence. Please quantify the refit after excluding the two dusty stars, report whether a statistically significant slope remains for [Fe/H] > −1.2, and clearly separate the dust-affected regime from a dust-free metallicity calibration. As written, the recommendation to apply a metallicity correction to metal-rich galaxies conflates extinction/emission by circumstellar dust with a change in TRGB luminosity.
- [Abstract] The abstract says 'for [Fe/H] > −1.2, MI is found to become fainter with lower metallicity', but Eq. (4) and the body text state the opposite: higher metallicity corresponds to a fainter (larger) MI. This is a sign error in the abstract and must be corrected to match the actual fit and discussion.
- [Section 3.1 and Figure 4] The four gray points are excluded because they deviate by 3σ from the KS-band fit, and then the same fit is reported as the final relation. This is mildly circular: the fit defines the outliers, and the outliers define the fit. The subsequent physical justification (Section 3.4: these stars are warmer and may not have reached the TRGB) is helpful, but the rejection was not originally based on that criterion. Please show the fits with and without each rejected point, demonstrate stability of the high-metallicity slope, and report how many of the six clusters with [Fe/H] > −1.2 drive the effect. Given that one of the four rejected clusters, NGC 6366, is in the high-metallicity regime, the robustness of the high-metallicity claim is not established.
- [Section 2 and Section 3.2] The identification of the TRGB as the single reddest star in the CMD is not validated for metal-rich clusters, where AGB contamination and variability are more common. The paper itself finds that 2 of 43 candidates are AGB stars and 4 are large-amplitude variables, and it reports that using the second-reddest star shifts MI by 0.029 mag. Please apply an edge-detection or luminosity-function method to at least a few clusters (e.g., NGC 5904 and a metal-rich cluster) to confirm that the reddest-star criterion indeed selects the RGB tip rather than an AGB or a dusty variable, and quantify how the selection bias varies with metallicity.
- [Section 3.3] The derived H0 = 70.86 ± 1.2 ± 0.9 km/s/Mpc is a direct algebraic transform of MI through Eq. 9 of Riess et al. (2016) with only the TRGB magnitude varied; it is therefore a restatement of the calibration and not an independent check. The comparison with the LMC predicted MI = −3.7 mag versus measured −4.04 mag is dismissed on the assertion that previous LMC TRGB measurements drew on metal-poor halo stars, but no evidence for that assertion is provided. Please either remove the H0 discussion or explicitly present it as a sensitivity illustration, and support the LMC metallicity argument with references or data.
minor comments (4)
- [Units] The Hubble constant units are written inconsistently as 'kms^{-1}Mpc^{-1}' in the abstract and text; please use consistent spacing, e.g., 'km s^{-1} Mpc^{-1}'.
- [Table 1] The notation for excluded stars (asterisk and plus sign) is explained only in the table caption; please clarify in the text of Section 2 that '∗' denotes LPV/AGB removal and '+' denotes the four 3σ outliers, and make clear that the final sample used for fitting has 33 stars.
- [Figure 7] The single deviant point NGC 6121 is visible in Figure 7 but not identified in the figure; please label it in the figure so the discussion in Section 3.5 is directly tied to the plot.
- [References] Cerny et al. (2020) is cited as 'arXiv' with no arXiv number; Bellazzini et al. (2004) is incomplete. Please complete the reference list for the journal submission.
Circularity Check
No significant circularity: the central M_I and H0 values are derived from external photometry, distances, and a standard transformation; the only mild self-referential step is a 3-sigma outlier rejection based on the fit itself, which is partly offset by an independent temperature check.
-
other
[Section 3.1, Figure 4 (text near 'In the KS band panel of Figure 4...')]
"In the KS band panel of Figure 4, there are four gray points that significantly deviate from the trend line. These outliers fall outside the 3 σ range of our fitting and are therefore excluded. The fitting in the other bands or intrinsic colors also eliminates these points."
The 3-sigma cutoff is computed from the residuals of the very fit that is being constructed; after removing the worst-fitting points, the same functional forms are refit to the surviving 33 stars. Thus the reported residuals and final relations (Eqs. 1-6) are not fully independent of the outlier-selection step. The loop is partially mitigated by the independent argument in Section 3.4 that the excluded stars are warmer and 'may not have reached the TRGB stage', but the statistical criterion itself remains fit-defined. This does not undermine the central metal-poor M_I anchor (-4.017), which is consistent with external calibrations, nor the H0 value, which is a direct transformation of M_I via the Riess et al. (2016) relation rather than a separately fitted input.
full rationale
The paper's main derivation chain is self-contained against external data: TRGB candidates are selected from Gaia DR3 photometry with membership probabilities from Vasiliev & Baumgardt (2021); absolute magnitudes are computed using distances from Baumgardt & Vasiliev (2021), reddening from Harris (2010), and an extinction law from Wang & Chen (2019). The M_I = -4.017 +/- 0.036 +/- 0.027 mag plateau at [Fe/H] < -1.2 is a fitted value, but it is not circularly defined: it is anchored to external photometric and astrometric measurements and agrees with independent previous calibrations (Freedman 2021; Dixon et al. 2023). The reported H0 = 70.86 +/- 1.2 +/- 0.9 km/s/Mpc is a direct application of the Riess et al. (2016) formula to this M_I, so it is a derived quantity rather than a prediction forced by the inputs. The only noticeable self-referential step is the 3-sigma outlier rejection in Section 3.1, where the fit defines which points are excluded before the final fit is made; this is a mild methodological loop but not a load-bearing one, especially because Section 3.4 supplies an independent temperature-based justification for excluding those stars. No load-bearing self-citation, no uniqueness theorem imported from the authors, and no renaming of known results were found. The paper's internal inconsistency about the sign of the high-metallicity trend (abstract vs. text) and the circumstellar-dust interpretation are correctness concerns, not circularity, and therefore do not raise the circularity score.
Assumptions & free parameters
free parameters (5)
- a0, a1, a2 in M_I fit (Eq. 4) =
10.47, 3.92, -4.017
- Coefficients for M_GBP, M_GRP, M_V, M_J, M_KS fits (Eqs. 1-3, 5-6) =
See Eqs. 1-6
- Coefficients for color-metallicity relations (Eqs. 7-9) =
See Eqs. 7-9
- LPV variability cutoff of 0.05 mag =
0.05 mag
- 3-sigma outlier rejection in the KS band =
3 sigma
assumptions (5)
- domain assumption Cluster distances from Baumgardt & Vasiliev (2021) are accurate.
- domain assumption Metallicities and reddening from Harris (2010) are correct.
- domain assumption The extinction law of Wang & Chen (2019) applies to all clusters.
- ad hoc to paper The reddest star in the CMD is the TRGB.
- domain assumption The SNe Ia slope ax in the Riess et al. (2016) H0 relation is unchanged.
Cite this review
Pith. "Pith review of Dependence of Multi-band Absolute Magnitudes and Color Indexes of the Tip of Red Giant Branch Stars on Metallicity in the Galactic Globular Clusters." pith.science (2026). https://pith.science/paper/CCMGLHEA
@misc{pith2026250203705,
author = {Pith},
title = {Pith review of: Dependence of Multi-band Absolute Magnitudes and Color Indexes of the Tip of Red Giant Branch Stars on Metallicity in the Galactic Globular Clusters},
year = {2026},
howpublished = {\url{https://pith.science/paper/CCMGLHEA}},
note = {Machine review of arXiv:2502.03705}
}
abstract
The tip of red giant branch (TRGB) stars have attracted intensive attention in recent years because their $I$-band absolute magnitudes, $M_\rm I$, are often used for distance calibration in the Hubble constant measurements because of its almost independence on metallicity ([Fe/H]). However, a discrepancy exists between various studies and the theoretical stellar model predicts dependence of their luminosity on [Fe/H]. Here we present a careful study of the dependence of absolute magnitudes and color indexes on metallicity in optical and near-infrared bands. With the TRGB stars identified in 33 Galactic globular clusters by the reddest color in the $G_{\rm BP}-G_{\rm RP}$ vs. $G_{\rm RP}$ diagram, it is confirmed that $M_\rm I$ is almost constant of $-4.017 \pm 0.036 \pm 0.027$ mag when $[\rm Fe/H]<-1.2$, which would give $H_0=70.86\pm 1.2\pm0.9$ $\rm kms^{-1} Mp c^{-1}$ with this updated luminosity calibration for type Ia supernovae. However, for $[\rm Fe/H]>-1.2$, $M_\rm I$ is found to become fainter with lower metallicity, which would lead to a larger Hubble constant. In the optical $G_{\rm BP}, G_{\rm RP}$ and $V$ bands, the absolute magnitude of TRGB stars tends to increase with metallicity, while in the infrared $J, H$, and $K_{\rm S}$ bands, the variation with metallicity shows an inverse tendency. In addition, the analytical relations of the color indexes with metallicity are presented, which have smaller dispersion than those derived for the corresponding absolute magnitudes.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[1]
, " * write output.state after.block = add.period write newline
ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts ...
-
[2]
write newline
" write newline "" before.all 'output.state := FUNCTION format.url url empty "" new.block "" url * "" * if FUNCTION format.eprint eprint empty "" archivePrefix empty "" archivePrefix "arXiv" = new.block " " eprint * " " * new.block " " eprint * " " * if if if FUNCTION format.doi doi empty "" " " doi * " " * if FUNCTION format.pid doi empty eprint empty ur...
-
[3]
thebibliography [1] 20pt to REFERENCES 6pt =0pt -12pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key on reference command E...
arXiv 2021
-
[4]
2022, , 259, 35, 10.3847/1538-4365/ac4414
Abdurro'uf , Accetta , K., Aerts , C., & Silva Aguirre . 2022, , 259, 35, 10.3847/1538-4365/ac4414
-
[5]
Anand, G. S., Tully, R. B., Rizzi, L., Riess, A. G., & Yuan, W. 2021, The Astrophysical Journal, 932
work page 2021
-
[6]
Anderson , R. I., Koblischke , N. W., & Eyer , L. 2024, , 963, L43, 10.3847/2041-8213/ad284d
-
[7]
2021, Monthly Notices of the Royal Astronomical Society, 505, 5957, 10.1093/mnras/stab1474
Baumgardt, H., & Vasiliev, E. 2021, Monthly Notices of the Royal Astronomical Society, 505, 5957, 10.1093/mnras/stab1474
-
[8]
Bellazzini, M., Ferraro, F. R., Sollima, A., et al. 2004, Astronomy and Astrophysics, 424, 199
work page 2004
Show all 45 references
-
[9]
2012, Monthly Notices of the Royal Astronomical Society, 427, 127
Bressan, A., Marigo, P., Girardi, L., et al. 2012, Monthly Notices of the Royal Astronomical Society, 427, 127
2012
-
[10]
2012, , 427, 127, 10.1111/j.1365-2966.2012.21948.x
Bressan , A., Marigo , P., Girardi , L., et al. 2012, , 427, 127, 10.1111/j.1365-2966.2012.21948.x
2012
-
[11]
L., Madore, B
Cerny, W., Freedman, W. L., Madore, B. F., et al. 2020, arXiv
2020
-
[12]
Choudhury, S., Subramaniam, A., & Cole, A. A. 2015, Monthly Notices of the Royal Astronomical Society, 455, 1855, 10.1093/mnras/stv2414
2015 doi
-
[13]
2020, A&a, 641, A12
Collaboration, P., Aghanim, N., Akrami, Y., et al. 2020, A&a, 641, A12
2020
-
[14]
M., Wright , E
Cutri , R. M., Wright , E. L., Conrow , T., et al. 2012, Explanatory Supplement to the WISE All-Sky Data Release Products , Explanatory Supplement to the WISE All-Sky Data Release Products
2012
-
[15]
2023, , 523, 2283, 10.1093/mnras/stad1500
Dixon , M., Mould , J., Flynn , C., et al. 2023, , 523, 2283, 10.1093/mnras/stad1500
2023 doi
-
[16]
L., Chaboyer, B
Dotter, A. L., Chaboyer, B. C., Jevremovi \'c , D., et al. 2008, The Astrophysical Journal Supplement Series, 178, 89
2008
-
[17]
Freedman, W. L. 2021, The Astrophysical Journal, 919
2021
-
[18]
L., Madore, B
Freedman, W. L., Madore, B. F., Hatt, D., et al. 2019, The Astrophysical Journal, 882
2019
-
[19]
L., Madore, B
Freedman, W. L., Madore, B. F., Hoyt, T. J., et al. 2020, The Astrophysical Journal, 891
2020
-
[20]
Gaia Collaboration , Vallenari , A., & Brown , A. G. A. 2023, , 674, A1, 10.1051/0004-6361/202243940
2023 doi
-
[21]
2018, The Astronomical Journal, 156
G'orski, M., Pietrzy'nski, G., Gieren, W., et al. 2018, The Astronomical Journal, 156
2018
-
[22]
2008, Astronomy and Astrophysics, 486, 951
Gustafsson, B., Edvardsson, B., Eriksson, K., et al. 2008, Astronomy and Astrophysics, 486, 951
2008
-
[23]
Harris, W. E. 2010, arXiv: Astrophysics of Galaxies
2010
-
[24]
J., Beaton , R
Hoyt , T. J., Beaton , R. L., Freedman , W. L., et al. 2021, , 915, 34, 10.3847/1538-4357/abfe5a
2021 doi
-
[25]
D., Pfeffer, J
Kruijssen, J. D., Pfeffer, J. L., Reina-Campos, M., Crain, R. A., & Bastian, N. 2019, Monthly Notices of the Royal Astronomical Society, 486, 3180
2019
-
[26]
V., Milone, A
Legnardi, M. V., Milone, A. P., Cordoni, G., et al. 2023, Monthly Notices of the Royal Astronomical Society, 522, 367, 10.1093/mnras/stad1056
2023 doi
-
[27]
Li , S., Casertano , S., & Riess , A. G. 2022, , 939, 96, 10.3847/1538-4357/ac7559
2022 doi
-
[28]
Li, S., Casertano, S., & Riess, A. G. 2023, The Astrophysical Journal, 950, 83, 10.3847/1538-4357/accd69
2023 doi
-
[29]
2024, , 167, 123, 10.3847/1538-3881/ad23e8
Li , Y., Jiang , B., & Ren , Y. 2024, , 167, 123, 10.3847/1538-3881/ad23e8
2024 doi
-
[30]
Ma \' z Apell \'a niz , J., Holgado , G., Pantaleoni Gonz \'a lez , M., & Caballero , J. A. 2023, , 677, A137, 10.1051/0004-6361/202346759
2023 doi
-
[31]
2017, , 835, 77, 10.3847/1538-4357/835/1/77
Marigo , P., Girardi , L., Bressan , A., et al. 2017, , 835, 77, 10.3847/1538-4357/835/1/77
2017 doi
-
[32]
2017, The Astrophysical Journal, 835
Marigo, P., Girardi, L., Bressan, A., et al. 2017, The Astrophysical Journal, 835
2017
-
[33]
McQuinn, K. B. W., Boyer, M. L., Skillman, E. D., & Dolphin, A. E. 2019, The Astrophysical Journal, 880
2019
-
[34]
G., Casertano, S., Yuan, W., et al
Riess, A. G., Casertano, S., Yuan, W., et al. 2021, The Astrophysical Journal Letters, 908, L6
2021
-
[35]
G., Macri , L
Riess , A. G., Macri , L. M., Hoffmann , S. L., et al. 2016, , 826, 56, 10.3847/0004-637X/826/1/56
2016 doi
-
[36]
2005, , 357, 669, 10.1111/j.1365-2966.2005.08689.x
Salaris , M., & Girardi , L. 2005, , 357, 669, 10.1111/j.1365-2966.2005.08689.x
2005
-
[37]
D., & Tognelli, E
Saltas, I. D., & Tognelli, E. 2022, Monthly Notices of the Royal Astronomical Society
2022
-
[38]
F., Cutri , R
Skrutskie , M. F., Cutri , R. M., Stiening , R., et al. 2006, , 131, 1163, 10.1086/498708
2006 doi
-
[39]
Soltis, J., Casertano, S., & Riess, A. G. 2020, The Astrophysical Journal Letters, 908
2020
-
[40]
G., Oelkers , R
Stassun , K. G., Oelkers , R. J., Paegert , M., et al. 2019, , 158, 138, 10.3847/1538-3881/ab3467
2019 doi
- [41]
-
[42]
2004, Monthly Notices of the Royal Astronomical Society, 354, 815
Valenti, E., Valenti, E., Ferraro, F., & Origlia, L. 2004, Monthly Notices of the Royal Astronomical Society, 354, 815
2004
-
[43]
2021, Monthly Notices of the Royal Astronomical Society, 505, 5978
Vasiliev, E., & Baumgardt, H. 2021, Monthly Notices of the Royal Astronomical Society, 505, 5978
2021
-
[44]
2019, The Astrophysical Journal, 877, 116, 10.3847/1538-4357/ab1c61
Wang, S., & Chen, X. 2019, The Astrophysical Journal, 877, 116, 10.3847/1538-4357/ab1c61
2019 doi
-
[45]
G., Macri, L
Yuan, W., Riess, A. G., Macri, L. M., Casertano, S., & Scolnic, D. M. 2019, The Astrophysical Journal, 886, 61, 10.3847/1538-4357/ab4bc9
2019 doi
Reviewed August 9, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.