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REVIEW 4 major objections 5 minor 57 references

Tracing Red Giant Members of the Globular Cluster Palomar 5 with APOGEE and Gaia

T0 review · 4 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read The absence of new red-giant members in APOGEE fields over Palomar 5 is explained by the survey's magnitude limit alone, not by new physics.

desk verdict A transparent, useful null result: Pal 5's missing APOGEE red giants are plausibly just below the survey's magnitude limit, though the expected-count estimate is a coarse global average rather than a per-field prediction. read the letter →

arxiv 2507.21212 v1 pith:HXIS56NL submitted 2025-07-28 astro-ph.GA

Allyson A. Sheffield , Sarah Pearson , Lenin F. Nolasco , Rachael L. Beaton , Adrian M. Price-Whelan , Katia Cunha , Verne V. Smith , Rafael Guerco This is my paper · ORCID
classification astro-ph.GA
keywords Palomar5globularclustertidalstreamsredgiantsAPOGEEGaiaastrometrychemicaltagginggalactichalostellarpopulations
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 why a search of five APOGEE spectroscopic fields found no new red-giant members of the tidally disrupted globular cluster Palomar 5, either in the cluster core or in its leading and trailing tidal streams. The authors recover eight previously known members (six in the core, two in the trailing arm) and then build a synthetic stellar population to estimate how many giants APOGEE should have seen in those fields. The estimate says few, if any, new giants are expected: roughly one to two per three-degree field for the brighter magnitude limit and two to three for the deeper limit, depending on stream density. The paper concludes that the empty result is consistent with the known stream density and APOGEE's limiting magnitude, and that it supports earlier evidence for density variations along the streams.

What carries the argument

The carrying object is the synthetic stellar population used for expected-count calculations: a stellar-evolution isochrone for an old, metal-poor population, shifted to Pal 5's distance, with a standard initial mass function, normalized so 3000 stars fall in the apparent magnitude range 20 < g0 < 23 over the stream. Counting how many synthetic giants fall below APOGEE's H-band limits (H0 = 13.8 and 14.5) turns the empty detection into a predicted number per pointing. A second mechanism is the chemical-tagging scheme, which uses the abundance loci of the eight recovered members in planes such as [C/Fe]-[N/Fe], [Mg/Fe]-[Al/Fe], and [Mn/Al]-[Fe/H] to flag candidates, followed by kinematic and color-magnitude vetting.

What would settle it

A deeper near-infrared spectroscopic survey of these same five fields reaching about H0 = 15 would settle it: the model predicts only a handful of giants, so finding several new Pal 5 members there would falsify the claim that the non-detection is purely a magnitude-limit effect.

Watch

Extended reading notes

Core claim

The paper's central claim is that the absence of new red-giant detections in the APOGEE pointings is a selection effect, not a dynamical anomaly. Using a synthetic population built from an 11.5 Gyr isochrone at [Fe/H] = -1.3, shifted to Pal 5's distance modulus of 16.6, the paper predicts roughly 6-13 stream giants brighter than H0 = 13.8 and 16-24 brighter than H0 = 14.5 across the entire 26-degree stream. Spread evenly, that yields about one to two giants per APOGEE pointing at the brighter limit and two to three at the deeper limit, and density variations along the stream could lower those numbers further. For the core, the same population with a present-day core mass near 4000 solar masses predicts about five giants at the brighter limit, matching the six already detected. The paper therefore argues that the non-detection requires no new physics and is consistent with known stream density variations.

Load-bearing premise

The calculation assumes the stream contains 3000 stars with apparent magnitude 20 < g0 < 23 spread uniformly over 26 degrees, and that APOGEE observed every giant brighter than its magnitude limit in these pointings; if the stream is richer or APOGEE missed bright giants, the expected number of detections rises.

Editorial extensions

If this is right

  • Future searches for Pal 5 giants at these stream locations will need deeper near-infrared spectroscopy than APOGEE's nominal limit if they aim to find new members.
  • The empty leading-arm field is consistent with either a truncated stream or a fanned, low-density extension that sits below APOGEE's detection threshold.
  • The density variations implied by the non-detection keep baryonic perturbers, dark-matter subhaloes, and passing globular clusters as viable explanations.
  • Recovering the eight known members with both a position/proper-motion box and HDBSCAN shows the selection method finds Pal 5 giants when they are present, strengthening the interpretation of the null result.
  • The consistency between the predicted and observed number of core giants supports a present-day core mass near the lower end of the estimated range.

Reading between the lines

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

  • A direct extension would be to point a deeper near-infrared spectrograph at the same five fields: the model predicts only a handful of giants, so finding several would immediately challenge the magnitude-limit explanation.
  • The same synthetic-population counting could be applied to other globular cluster streams that overlap APOGEE pointings, turning null detections into quantitative constraints on stream density and survey completeness.
  • The normalization of 3000 stars in the 20 < g0 < 23 window is the main lever; improved photometric stream counts along the full 26 degrees would tighten the prediction and test whether the assumed uniform spread is realistic.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

4 major / 5 minor

Summary. This paper searches for red giant members of the globular cluster Palomar 5 in five APOGEE DR17 fields, using Gaia DR3 astrometry and APOGEE chemical abundances. The authors identify eight reference members (six in the core, two in the stream) by position and proper motion, and recover the same eight with HDBSCAN clustering. They then perform chemical tagging in the [C/Fe]-[N/Fe] plane, with additional abundance planes as checks, and select several dozen candidates, which are screened using the color-magnitude diagram and kinematics. The final result is a null detection of new members. To interpret this, the authors compute the expected number of APOGEE giants in the stream fields using a synthetic stellar population normalized to 3000 stars with 20<g0<23 over a 26-degree stream, and conclude that only 1-2 (H0<13.8) or 2-3 (H0<14.5) giants are expected per 3-degree field. They conclude that the non-detection is consistent with APOGEE's magnitude limit and possibly with density variations in the streams.

Significance. If the expected-count calculation is correct, the paper provides a useful demonstration that a non-detection of Pal 5 giants in APOGEE is unsurprising and that the search does not require new physics. The work makes good use of public data and includes a careful comparison with existing literature, orbit integrations, and a discussion of stream density perturbations. However, the expected-count calculation rests on a uniform stream density and an unquantified assumption of APOGEE completeness, and the statistical power of the null detection is low. The paper's contribution is therefore modest but potentially publishable after the quantitative argument is strengthened.

major comments (4)
  1. [Section 5.1] The expected-count calculation spreads the total number of stream giants uniformly over the 26-degree stream and then derives a per-field expectation of 1-2 or 2-3 stars per 3-degree pointing. The two APOGEE stream fields are located at the extremes of the known stream, where the linear density is known to be lower than the average (Erkal et al. 2017, Figures 7 and 9; Bonaca et al. 2020; Kuzma et al. 2022). Because the conclusion that the non-detection is 'simply due to the limiting magnitude of the survey' depends directly on this per-field expectation, the calculation should be repeated using the stream density at the specific phi1 of each pointing, and a Poisson error bar should be included. Without this, the claim 'few, if any, new giants are expected' is not quantitatively supported for these fields.
  2. [Section 5.1] The expected-count calculation implicitly assumes that APOGEE observed every giant brighter than H0=13.8 (or 14.5) in its pointings. APOGEE target selection uses color and magnitude cuts, and finite fiber allocation means the completeness is not 100%; this completeness is never quantified for the five fields. If the completeness is less than unity, the expected number of detected giants is lower, which would make the non-detection even less statistically significant. The authors should either state the assumed completeness and its source or present the expected counts as upper limits and discuss how completeness affects the interpretation of the null result.
  3. [Section 5.1, core mass consistency check] For the core consistency check, the calculation predicts 15 giants for Mcore=16,000 Msun at the H0<13.8 limit and 33 at the H0<14.5 limit, whereas only six giants are observed. The paper notes that the lower mass case is consistent but does not discuss the discrepancy for the higher mass case. This discrepancy either implies the core mass is at the low end of the estimated range or that the synthetic population normalization over-predicts the number of giants. In either case, it is directly relevant to the reliability of the stream expectation and should be addressed explicitly.
  4. [Abstract and Section 6] The conclusion that 'Our findings support the presence of density variations along the Pal 5 streams' is stronger than the data warrant. With an expected yield of 1-2 or 2-3 giants per field, the probability of detecting zero in the two stream fields is roughly 20% (lambda=1.5) to 8% (lambda=2.5), so the non-detection is only weakly consistent with a density deficit and does not provide significant support for density variations. The conclusion should be revised to state that the non-detection is consistent with the magnitude limit and possible low density at the stream ends, but is not statistically significant.
minor comments (5)
  1. [Abstract] The sentence 'Its core is currently at a heliocentric distance of ~21 kpc, near apogalacticon (~18 kpc)' is internally confusing: a heliocentric distance of 21 kpc cannot be near an apogalacticon of 18 kpc. Clarify whether the two distances refer to heliocentric and Galactocentric frames, or correct the orbital phase description.
  2. [Section 2.2] The proper motion box width of ±0.2 mas/yr is adopted without justification. Stream stars at larger phi1 can have proper motions that deviate from the cluster mean (as shown in Figure 5), so the eight reference stars may be biased toward the cluster core. The authors should state whether the results are robust to a wider proper motion box.
  3. [Section 3.2] The chemical tagging is described as requiring only the [C/Fe]-[N/Fe] box, with the other three abundance planes used as a visual 'check.' This makes the selection procedure not fully reproducible. Consider formalizing the criteria (e.g., requiring agreement in a specified number of planes) or clearly stating that the final membership assessment is qualitative.
  4. [Figure 2] The numbering scheme in the right panel of Figure 2 shows 27 numbers for 26 core candidates because one star was observed twice. The caption should state this explicitly to avoid confusion.
  5. [Section 5.1] The paper uses two H-band limits, 13.8 and 14.5, and notes that 13.8 'may be more realistic,' but the main text does not clearly state which limit is used for the headline expectation. The choice should be stated explicitly, since the expected counts differ by roughly a factor of two between the two limits.

Circularity Check

1 steps flagged · score 2.0 of 10

Only minor circularity in the HDBSCAN verification step; the central expected-count argument is a forward model with independent, externally falsifiable inputs.

  1. fitted input called prediction [Section 2.2, HDBSCAN membership identification paragraph]
    "We follow the recommendation by Campello et al. (2013) and adopt the same value for both parameters, setting m Pts = m clSize = 2. This choice yielded the lowest standard deviation in metallicity among the clustering outcomes. We find the same eight stars as above, all with 100% probability of membership, when selecting on proper motion and the abundance ratios [Fe/H], [Mg/Fe], [N/Fe], [Al/Fe] and [Mn/Al]."

    The HDBSCAN minimum-cluster-size parameter is selected by inspecting the clustering outcomes and choosing the value that minimizes metallicity scatter, while [Fe/H] is itself one of the input features to the clusterer. The eight reference stars were already isolated by position and proper motion and form a compact, low-scatter metallicity group, so tuning mclSize to minimize metallicity scatter effectively selects for an output containing that group. Presenting 'HDBSCAN finds these same eight' as an independent confirmation is therefore a fitted parameter renamed as a predicted finding. However, this step is used only as a verification check; the paper's conclusion about expected APOGEE giants does not depend on the HDBSCAN result.

full rationale

The central claim, that few or no new Pal 5 red giants are expected in the APOGEE pointings because of the survey's magnitude limit, is computed as a forward model: a PARSEC isochrone and Kroupa IMF are combined with a photometric normalization of 3000 stream stars in 20<g0<23 taken from Pearson et al. (2019) and Bonaca et al. (2020), and then stars with H0<13.8 or H0<14.5 are counted and divided over the stream length. This is an extrapolation from published, externally falsifiable star counts; it is not a re-derivation of the input, and the cited prior work does not assume the conclusion of non-detection. The self-citation is present (Pearson and Price-Whelan are co-authors of the cited works) but the normalizing star count is an empirical input, not a uniqueness theorem or an ansatz, so it does not make the derivation circular. The HDBSCAN parameter tuning is a genuine but minor circularity because the confirmation is selected for rather than independently discovered. The lack of a per-field density and completeness error budget in the expected-count calculation is a correctness risk, but it is not a circularity. Overall the paper's main result is not forced by its inputs, and the mild self-fitting in one verification step places it at a low circularity score.

Assumptions & free parameters 7 free parameters · 6 assumptions · 0 invented entities

The central expectation estimate imports several external inputs from prior studies, most importantly the 3000-star normalization, the 26-degree stream length, and the isochrone/IMF model. No invented entities are introduced. The APOGEE completeness assumption is the most fragile entry because it is an unstated condition on the expected-count calculation.

free parameters (7)
  • HDBSCAN mPts and mclSize = 2
    Chosen with mPts = mclSize = 2 after verifying this yields the lowest standard deviation in metallicity for the recovered cluster, i.e., tuned to produce a compact mono-metallic group. Used for the member-finding cross-check only.
  • Proper motion selection box width = ±0.2 mas/yr
    Applied around the mean Pal 5 proper motion from Vasiliev & Baumgardt (2021) to define the eight reference stars. This hand-chosen width may exclude genuine stream members with larger PM deviations, but it is used to establish the chemical baseline rather than for the final search.
  • Metallicity range for chemical search = -1.5 < [Fe/H] < -1.0
    Applied to restrict chemical tagging candidates to a mono-metallic GC range. Could exclude genuine Pal 5 members outside this range.
  • C-N chemical selection box = Pink box in Figure 3
    Drawn around the eight reference stars using typical GC [C/Fe] and [N/Fe] ranges from Mészáros et al. (2020). This defines which stars are flagged as chemical candidates.
  • Stream length for expected-count spreading = 26 degrees
    Adopted from known Pal 5 detections spanning -18 < phi1 < 8 deg. Used with total expected stars to derive linear density.
  • Synthetic population normalization = 3000 stars with 20 < g0 < 23
    Taken from Pearson et al. (2019) and Bonaca et al. (2020). The predicted APOGEE yield scales linearly with this number, so it is the load-bearing input for the 'few, if any, new giants' claim.
  • Core mass assumptions = 4,000 and 16,000 Msun
    Used in the consistency check for the expected number of APOGEE giants in the core, bracketing the Ibata et al. (2017) mass estimates.
assumptions (6)
  • domain assumption A PARSEC isochrone with age 11.5 Gyr and [Fe/H] = -1.3 represents Pal 5's stellar population on the CMD and in the synthetic luminosity function.
    Used in Figures 2 and 7 and in Section 5.1 to map absolute magnitude to apparent magnitude. Pal 5's exact age and metallicity are not independently derived here.
  • domain assumption The synthetic stellar population follows a Kroupa IMF and produces an average stellar mass of about 0.83 Msun.
    Used in Section 5.1 to convert core mass to number of stars. If the IMF is different, the expected giant counts change.
  • domain assumption The Pal 5 stream spans about 26 degrees (from -18 to 8 degrees in phi1) with the same stellar population as the core.
    Used to spread the expected number of APOGEE giants across the stream. Prior detections define this extent; the stream may extend beyond it.
  • domain assumption APOGEE observed every giant star with H0 < 13.8 or 14.5 within its 3-degree pointings.
    Assumed implicitly in Section 5.1 when converting expected stars per degree to expected stars per pointing. APOGEE target selection and fiber assignment are not modeled, so the true completeness is unknown.
  • domain assumption The mean proper motion of Pal 5 from Vasiliev & Baumgardt (2021), (-2.73, -2.66) mas/yr, is accurate and representative of the cluster and inner stream.
    Used for reference-star selection in Section 2.2 and for kinematic comparisons in Section 4.
  • domain assumption A distance modulus of 16.6 places the cluster at 20.6 kpc, with stream stars allowed to scatter in the CMD by up to ~0.7 mag from distance gradients.
    Used to shift the isochrone and to interpret candidates that fall above the giant branch as closer stream stars.

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Cite this review

Pith. "Pith review of Tracing Red Giant Members of the Globular Cluster Palomar 5 with APOGEE and Gaia." pith.science (2026). https://pith.science/paper/HXIS56NL

@misc{pith2026250721212,
  author       = {Pith},
  title        = {Pith review of: Tracing Red Giant Members of the Globular Cluster Palomar 5 with APOGEE and Gaia},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/HXIS56NL}},
  note         = {Machine review of arXiv:2507.21212}
}
read the original abstract

The globular cluster Palomar 5 (Pal 5) is in the process of being tidally shredded as it orbits the Milky Way. Its core is currently at a heliocentric distance of ~21 kpc, near apogalacticon (~18 kpc), and it reaches ~5-7 kpc at perigalacticon. Pal 5's leading and trailing arms stretch over 20 degrees on the sky, making them sensitive probes of the Milky Way's mass distribution. In this work, we search for red giant members of Pal 5 using spectroscopic data from APOGEE DR17 and photometric and astrometric data from Gaia DR3. Based on position and proper motion, we identify eight members of Pal 5: six in the core and two in the stream. The clustering algorithm HDBSCAN finds these same eight. We then use chemical tagging with APOGEE abundances to search for additional members across five APOGEE fields overlapping Pal 5. While several dozen candidates are identified, most deviate (some significantly) from known kinematic and color-magnitude trends, suggesting that they are less likely to be true members. We estimate the expected number of giants in the APOGEE pointings based on the area and stellar mass of the streams. Given APOGEE's limiting magnitude, we find that few, if any, new giants are expected, especially if the stream is more diffuse at these locations. Our results support the presence of density variations in Pal 5's tidal streams, consistent with earlier studies attributing such features to baryonic perturbers in the Milky Way, dark matter subhaloes, or interactions with passing globular clusters.

Figures

Figures reproduced from arXiv: 2507.21212 by the authors.

Figure 1
Figure 1. Sky coordinates for the five APOGEE pointings used to identify possible Pal 5 members (black points). The gray points show other pointings nearby, which we did not analyze. Pal 5 members from the literature are shown as follows: Kuzma et al. (2022) (orange triangles), Price-Whelan et al. (2019) (magenta pentagons), Phillips et al. (2022) (green squares) and Ibata et al. (2017) (cyan plus sign). The cyan plus sign ma… view at source ↗
Figure 2
Figure 2. Color-magnitude diagram with Gaia photometry. Left Panel: stars in the five APOGEE pointings from [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. APOGEE DR17 abundances for stars with −1.5 <[Fe/H]< −1.0 in the five APOGEE Pal 5 pointings (black points). The eight Pal 5 reference stars are show as the blue circles (note that there are 9 reference points in blue, as one of the stars was observed twice in APOGEE). We show error bars for these stars in the upper left panel and these error bars are representative of the APOGEE stars in our sample. To be considered… view at source ↗
Figures from the paper (9 more)
Figure 4
Figure 4. Figure 4 [PITH_FULL_IMAGE:figures/full_fig_p008_4.png]
Figure 5
Figure 5. Figure 5: Spatial and kinematic properties of the Pal 5 candidates in the leading arm identified via chemical tagging (see Section 3.2). The candidates in the leading arm field are indicated by the red numbered squares and the trailing arm candidates are shown as the green numbe…
Figure 6
Figure 6. Figure 6: Spatial and kinematic properties of the Pal 5 candidates in the core identified via chemical tagging (see Section 3.2). The candidates in the core fields are indicated by the teal numbered squares, and are also shown in the CMD ( [PITH_FULL_IMAGE:figures/full_fig_p011…
Figure 7
Figure 7. Figure 7: Left: 2MASS color-magnitude diagram shifted to the distance of Pal 5. The black points show stars in the five APOGEE pointings explored in this work. The H-band limit of 13.8 for APOGEE is indicated as the red horizontal dashed line and the H-band limit of 14.5 is indi…
Figure 8
Figure 8. Figure 8: Orbits for the eight Pal 5 reference stars. Each color represents a different star [PITH_FULL_IMAGE:figures/full_fig_p014_8.png]
Figure 9
Figure 9. Figure 9: Orbit of 2M14481587-0742550 (leading arm candidate 3) [PITH_FULL_IMAGE:figures/full_fig_p014_9.png]
Figure 10
Figure 10. Figure 10: Orbit of 2M14534182−0656222 (leading arm candidate 14) [PITH_FULL_IMAGE:figures/full_fig_p014_10.png]
Figure 11
Figure 11. Figure 11: Orbit of 2M15141111+0030487 (core candidate 6) [PITH_FULL_IMAGE:figures/full_fig_p015_11.png]
Figure 12
Figure 12. Figure 12: Orbit of 2M15203549+0056416 (core candidate 22) [PITH_FULL_IMAGE:figures/full_fig_p015_12.png]

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