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Puzzling radial gradients of K-band absorption features in the giant elliptical galaxy M87

T0 review · 2 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read New K-band data for M87 show that stellar-population models, given optical-derived parameters, reproduce only the Na and Ca indices — CO, Mg, and Fe are mismatched, with CO2.30 flat at ~13 Å out to half an effective radius.

desk verdict Careful, genuinely new K-band radial gradients for M87 with a clean model-prediction benchmark, but the headline claim that models match only Na and Ca rests on a hand-adopted [C/Fe] and independent ±1σ shifts that ignore covariances. read the letter →

arxiv 2506.05986 v1 pith:FCUTILNX submitted 2025-06-06 astro-ph.GA

F. La Barbera , A. Vazdekis , A. Pasquali , J. Heidt , E. Eftekhari , M.A. Beasley , A. Gargiulo , S. Bisogni
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C. Spiniello L.P. Cassarà M. Sarzi
This is my paper
classification astro-ph.GA
keywords stellarpopulationsK-bandspectroscopyabsorption-lineindicesCOabsorptionradialgradientsellipticalgalaxiesinitialmassfunctionM87
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

The paper uses new K-band spectra of the giant elliptical galaxy M87, taken with the LUCI spectrograph on the Large Binocular Telescope and binned in radius out to about half the galaxy's effective radius, to test whether stellar-population models can predict near-infrared line strengths once the stellar content is fixed by optical data. The answer is mostly no: feeding the models with M87's optical-derived age, metallicity, initial-mass-function slope, and element abundances reproduces only the Na and Ca indices, while CO, Mg, and Fe features are mispredicted. The most conspicuous failure is CO absorption: the CO2.30 index stays at ~13.2 Å, flat from the centre out to 0.5 Re, far above model predictions, even though M87's inner regions have a bottom-heavy (dwarf-dominated) stellar mass distribution that models say should suppress CO, and even though the same models match the weaker CO of the M31 bulge observed with the same instrument. If the result stands, current near-infrared stellar-population models cannot yet be trusted to constrain the star content of massive galaxies, and massive ellipticals appear to contain a population of CO-strong cool giants that models, and M31, lack.

What carries the argument

The load-bearing procedure is an extrapolation test. Stellar-population parameters for M87 are fixed by optical MUSE spectroscopy (age ~14 Gyr, metallicity declining from about +0.2 to about -0.1 dex, low-mass IMF slope and [Na/Fe] from S18), supplemented by hand-assigned abundance ratios for elements the optical does not constrain ([C/Fe]=0.2 dex, [O/Fe]=[Mg/Fe]+0.1, [Si/Fe]~[Mg/Fe], [N/Fe]~[Mg/Fe], [Ti/Fe]~[Ca/Fe]~0); these are fed into EMILES-BaSTI and CvD18 simple-stellar-population models, and the predicted K-band indices are compared to the observed ones at each radius without any fitting. The second mechanism is differential: M31's bulge was observed with the same instrument and setup, so any systematic error in the models should affect both galaxies, isolating any true stellar-population difference between M31 and a massive elliptical. The key observable is the CO2.30 index (central passband 22915–23015 Å with pseudocontinua at 22850–22895 and 23090–23170 Å), because CO absorption is carried by cool giants and therefore responds inversely to the low-mass IMF slope.

What would settle it

A single observation settles it: extend the LUCI K-band spectra of M87 beyond half an effective radius (and, ideally, to one effective radius) with S/N high enough to measure CO2.30. If the index falls from about 13 Å toward the 10–11 Å model prediction as [Mg/Fe] and the IMF slope decline, the flat-CO claim fails; if it stays near 13 Å, the CO-strong population is real and smooth, and the burden shifts to the models.

Watch

Extended reading notes

Core claim

The core result is empirical and model-independent in its first half, comparative in its second. In M87, eight K-band absorption indices — MgI2.10, MgI2.28, FeI2.23, FeI2.24, NaI2.21, CaI2.26, CO2.30, and CO2.32 — are measured from radially binned spectra smoothed to a common velocity dispersion of 320 km/s; all are flat with radius except NaI2.21, which declines outward with slope $-0.45 \pm 0.15$, matching the slope of the M31 bulge but offset to stronger values. Then, instead of fitting the K-band data, the authors forward-predict the indices using stellar-population parameters (age, metallicity, low-mass IMF slope, and abundance ratios) taken from optical MUSE spectroscopy, and compare those predictions with the observations at each radius. The comparison matches NaI2.21 and CaI2.26 for both galaxies, underpredicts both Mg indices for M87 and M31, and underpredicts CO2.30 (at ~13.2 Å, with no significant gradient) and CO2.32 for M87 only. Because the same models do match M31's CO, the discrepancy is not a global failure of the models; the authors conclude that massive ellipticals such as M87 host a CO-strong stellar component present at all probed radii that is absent from current models and from M31.

Load-bearing premise

The conclusions assume that the age, metal content, star-mass distribution, and chemical abundances read off the optical spectra of M87 and M31 — plus the extra assumptions about carbon, oxygen, silicon, nitrogen, titanium, and calcium — truly describe these galaxies, so that any gap between predicted and observed near-infrared lines must be the models' fault rather than the input parameters' fault.

Editorial extensions

If this is right

  • K-band CO and Mg features cannot yet be used as reliable IMF or abundance diagnostics for massive early-type galaxies, because current models mispredict them even when the stellar populations are fixed by optical data.
  • The CO excess is not caused by the bottom-heavy central IMF, by intermediate-age AGB stars, or by a concentration of young stars, since it persists, flat, out to 0.5 Re where the IMF is near-Milky-Way.
  • The M87–M31 offset shifts the problem from 'models are wrong globally' to 'massive ellipticals host a CO-strong component that M31 lacks,' because the same models match M31's CO.
  • The underpredicted Mg indices for both galaxies indicate that the models' sensitivity of MgI2.10 and MgI2.28 to [Mg/Fe] is too weak.
  • The negative radial gradient of NaI2.21 can be explained by the combined optical-derived gradients of metallicity, IMF slope, and [Na/Fe], without invoking new physics.

Reading between the lines

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

  • If M87's CO-strong component is a fossil of its early formation, then other very massive galaxies formed in the same way should show the same flat CO excess; K-band IFU stacks of a sample of brightest cluster galaxies out to one effective radius would test whether the CO2.30 offset scales with galaxy mass or formation redshift.
  • The two hypotheses — extra carbon abundance versus a genuinely different cool-giant population — can be separated by line ratios: the CO2.32-to-CO2.30 ratio and the 12CO/13CO isotopic bands respond differently to carbon enhancement than to a cooler or more numerous giant branch.
  • The flat CO profile implies the responsible stars trace the old stellar light; deep NIR spectroscopy of globular clusters around M87, which share its old age and metallicity, could reveal whether the CO-strong population is a galaxy-wide property rather than a central anomaly.
  • Better cool-star model atmospheres below ~5000 K, or empirical libraries with more metal-rich giants, might close the CO gap without invoking a new population; the M31-vs-M87 offset weakens but does not exclude this, and a quantitative re-fit of model sensitivities to [C/Fe] and [O/Fe] would sharpen the test.
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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

2 major / 5 minor

Summary. F. La Barbera et al. present new K-band (2.02–2.36 µm) long-slit spectroscopy of the giant elliptical galaxy M87, obtained with LUCI1/2 at the LBT along the major axis. After telluric/sky correction and Monte-Carlo uncertainty estimation, spectra are binned radially to S/N ≥ 90 Å⁻¹ out to ~0.5 Re, excluding the central 5″ to avoid jet/AGN contamination. Eight Lick-like indices (MgI2.10, MgI2.28, FeI2.23, FeI2.24, NaI2.21, CaI2.26, CO2.30, CO2.32) are measured after smoothing data and models to a common σ = 320 km s⁻¹. The empirical results are: flat radial profiles for most M87 indices, a ~3σ negative NaI2.21 gradient, systematically higher line strengths than in the M31 bulge observed with the same instrument, and flat, strong CO2.30 ≈ 13.2 Å, consistent with stacked massive-ETG spectra but well above EMILES and CvD18 predictions. Rather than fitting NIR models, the authors build EMILES-BaSTI prediction bands from optical parameters (S18 for M87; LB21/LB24 for M31), shifting each parameter independently by ±1σ, and conclude that models match only the Na and Ca indices of M87 while underpredicting Mg and CO. They speculate that massive ETGs host a CO-strong stellar population absent from current models.

Significance. The paper's empirical contribution is strong: it provides the first K-band radial-gradient measurements for a giant elliptical beyond the nuclear region, with careful reduction (Monte-Carlo uncertainties, slit-side and observing-epoch consistency checks, exclusion of the contaminated central bin) and a same-instrument M31 comparison. The empirical content (flat strong CO, negative NaI2.21 gradient, M87–M31 offsets) is model-independent and provides a falsifiable benchmark for NIR stellar population models, and the chosen methodology (predict from optical parameters rather than fit the NIR data) is sound and free of circularity. If the model-comparison result survives a proper treatment of parameter covariances, the paper sharpens a genuinely open problem in NIR modeling of old stellar populations. The main caveat is that the headline 'only Na and Ca' claim inherits the assumptions of §5.2; the stress-test concern about the adopted [C/Fe] prior and the ignored covariances is real, though it does not affect the paper's empirical results.

major comments (2)
  1. [§5.2, Fig. 4] The abstract's headline claim that models 'are able to match only the Na and Ca indices of M87' rests on the prediction bands built in this section, and the stress-test concern about their construction is justified. The adopted [C/Fe] = 0.2 dex, described as 'consistent with the best-fitting results of S18', is an adopted input rather than a direct optical measurement, and its assumed 0.05 dex uncertainty is asserted without derivation; this matters because CO2.30 increases with [C/Fe] in the models (§5.1.5). In addition, the band is generated by shifting each parameter independently by ±1σ and taking the min/max of the resulting index values, which neglects covariances among age, [Z/H], Γb, [Mg/Fe], [Na/Fe], and [C/Fe] in the S18 posterior; the min–max envelope is not a confidence region on the prediction. The claim in §5.2 that the conclusions are 'not affected significantly' by the uncertain abundance-ratio assumptions is asserted rather than demonstrated. Please recompute the bands from the full S18 posterior or a joint covariance model, report the per-bin CO2.30 and CO2.32 prediction ranges, and quantify the significance of the observed values relative to the revised bands. If correlated excursions in the CO-enhancing direction (higher [C/Fe] and [Z/H], lower Γb) can bring CO2.30 within ~1σ of the data, the abstract's wording should be softened.
  2. [§5.2, Fig. 4] The construction of the prediction bands is not reproducible as written. Section 3 describes EMILES models with variable [Na/Fe] (NAMILES) but does not state that EMILES can vary [C/Fe], [O/Fe], [Si/Fe], [N/Fe], [Ca/Fe], or [Ti/Fe]; yet the caption of Fig. 4 and the text of §5.2 state that EMILES-BaSTI predictions are computed with abundance ratios varied according to optical results, assigning 0.05 dex uncertainties to all elements except Na. Please specify exactly which model version and which machinery (for example, CvD18 response functions applied to EMILES spectra, or a new EMILES release) is used to vary each element, and provide the response of each index to each adopted abundance ratio at the reference parameters. Note also that the updated EMILES models used here are described only as 'in preparation' (Vazdekis et al.); a public release or a full numerical description of the model spectra is needed for the Fig. 4 bands to be checkable.
minor comments (5)
  1. [§7, first bullet] The summary states that all K-band absorptions of M87 are flat except NaI2.21, but §5.1.5 and Table 2 report ∇CO2.32 = −0.90 ± 0.42 for M87, a marginally (≈2σ) negative gradient; the summary should be qualified.
  2. [Abstract] The phrase 'based on results from the optical spectral range' overstates the provenance of the prediction inputs, since [C/Fe] = 0.2 ± 0.05 dex is adopted rather than measured; consider rewording to 'optical-derived parameters supplemented by adopted abundance ratios'.
  3. [§2.1.2] 'V arizance maps' should read 'Variance maps'.
  4. [Appendix A] In the quadrature-correction sentence, the LUCI instrumental resolution is labelled with the symbol σEMILES = 35 km s⁻¹, where σLUCI or σinst is intended.
  5. [§5.2] The discussion of Na and Ca predictions states that these match 'reasonably well the data, except for the outermost radial bin'; given the sky/telluric concerns raised for that bin, the authors should state explicitly whether the conclusions of this paragraph change when the outermost bin is excluded.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the K-band indices are new measurements, and the model predictions are generated from optical-derived parameters and independent models rather than fitted to the K-band data.

full rationale

The paper's central empirical results—flat radial profiles for most K-band indices, the negative NaI2.21 gradient, and the M87 versus M31 offsets—are direct measurements of new LUCI spectra, not outputs of any model fit. The model comparison in Section 5.2 takes stellar population parameters (age, metallicity, IMF slope, abundance ratios) from the optical analyses of S18 for M87 and LB21/LB24 for M31, plus literature values from Conroy, Graves & van Dokkum (2014), and computes EMILES and CvD18 predictions without adjusting those parameters to match the K-band indices. The adopted [C/Fe]=0.2 dex is an input assumption with a stated uncertainty, not a quantity derived from the target CO indices; the paper explicitly acknowledges it as an assumption, and even if its uncertainty were underestimated, that would be a robustness or accuracy concern, not circularity. The comparison is also benchmarked against the external CvD18 models, so the claimed CO and Mg mismatches do not reduce to the authors' own models. Self-citations such as S18, LB17, LB24, and EVL21 are prior independent results based on different data sets, and the updated EMILES models are corroborated by CvD18 predictions. No equation in the paper defines a predicted quantity in terms of the observed K-band quantity, and no fitted K-band parameter is renamed as a prediction. The derivation chain is therefore self-contained: new data are measured, independent optical constraints are propagated through external models, and the residuals are reported as mismatches.

Assumptions & free parameters 3 free parameters · 5 assumptions · 1 invented entities

The central claims do not rest on any new derived constants or fitting of model parameters to the new K-band data. The free parameters listed are adopted abundance ratios used to build model predictions from optical-derived parameters; the paper argues the qualitative conclusions are robust to reasonable variations. The axioms are standard domain assumptions about data calibration, the validity of optical-derived stellar population parameters, and the representative nature of the two SSP models used as benchmarks. One speculative entity, a CO-strong stellar population, is introduced without independent evidence.

free parameters (3)
  • [C/Fe] for M87 = 0.2 dex (assumed radially constant)
    Adopted in Section 5.2 to compute model predictions for CO-sensitive indices. The paper argues a C abundance difference cannot explain the M87-M31 CO offset, but the value enters the predicted CO strength.
  • [O/Fe] offset relative to [Mg/Fe] = +0.1 dex
    Assumed in Section 5.2 for M87 and M31 following Conroy et al. (2014). Affects the predicted Mg and Fe indices and the conclusion that Mg responses are underestimated in the models.
  • Additional abundance ratios ([Si/Fe], [N/Fe], [Ti/Fe], [Ca/Fe]) = [Si/Fe]=[Mg/Fe], [N/Fe]=[Mg/Fe] for M87 or [O/Fe] for M31, [Ti/Fe]=0, [Ca/Fe]=0
    Hand-adopted in Section 5.2. The paper states K-band indices depend only weakly on most of these ratios, so the impact on the main conclusions is limited.
assumptions (5)
  • domain assumption Telluric and sky subtraction leave no significant residual systematics in the measured indices
    Section 2.1.2 describes consistency checks (opposite sides, different periods) but cannot fully exclude wavelength-dependent residuals; all line-strength measurements rest on this.
  • domain assumption The stellar population parameters of M87 from MUSE-based S18 analysis are correct at each radius
    Section 5.2 uses S18 age, metallicity, IMF slope, [Na/Fe], and [alpha/Fe] radial trends to generate K-band predictions; any systematic error in S18 propagates into the comparison.
  • domain assumption EMILES (with IRTF-extended library) and CvD18 are the state-of-the-art models for NIR SSP predictions
    The benchmark comparison assumes these models are representative of current model capabilities; the conclusion that they fail in K band is the paper's finding.
  • domain assumption K-band indices are insensitive to age for old stellar populations
    Invoked in Section 2.2 (citing Eftekhari et al. 2022a) to justify comparing M87 and M31 despite different formation timescales.
  • domain assumption [C/Fe] for M87 is radially constant at 0.2 dex
    Assumed in Section 5.2; the paper notes uncertainty in this assumption but argues it cannot produce the observed CO offset between M87 and M31.
invented entities (1)
  • CO-strong stellar population in massive ETGs
    purpose: Proposed in Section 6.2 as a possible explanation for the K-band CO excess in M87 and other massive ellipticals that is absent from current stellar population models.
    The paper offers no quantitative properties (temperature, luminosity, mass range) or new falsifiable prediction for this population beyond the already-measured CO excess, so it is a speculative placeholder rather than a defined entity.

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Cite this review

Pith. "Pith review of Puzzling radial gradients of K-band absorption features in the giant elliptical galaxy M87." pith.science (2026). https://pith.science/paper/FCUTILNX

@misc{pith2026250605986,
  author       = {Pith},
  title        = {Pith review of: Puzzling radial gradients of K-band absorption features in the giant elliptical galaxy M87},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FCUTILNX}},
  note         = {Machine review of arXiv:2506.05986}
}
read the original abstract

We present new K-band spectroscopy for the giant elliptical galaxy M87 in the Virgo cluster, taken with the LUCI spectrograph at the Large Binocular Telescope (LBT). The new data are used to study line-strengths of K-band absorption features from different chemical species, namely Fe, Mg, Ca, Na, and CO, as a function of galactocentric distance, out to 40arcsec from the center (about half of the galaxy effective radius). The radial trends of spectral indices are compared to those for the bulge of M31, observed with the same instrument. For M87, most K-band indices exhibit flat radial profiles, with the exception of NaI2.21, that decreases outwards, with a negative radial gradient. Significant offsets are found between indices for M87 and those for the bulge of M31, the latter having weaker line-strengths for almost all features, but Fe and Ca, for which we find similar trends in both systems. We find that the behavior of CO features - most prominent in giant stars - is difficult to explain, consistent with previous results for the central regions of massive galaxies. In particular, the CO indices are stronger in M87 than M31, and do not exhibit significant radial gradients in M87, despite its IMF being bottom heavier than M31 especially in its central region. Predictions of state-of-the-art stellar population models, based on results from the optical spectral range, are able to match only the Na and Ca indices of M87, while a significant mismatch is found for all other indices. This shows that state-of-the-art stellar population models should be improved significantly in order to provide reliable constraints on the stellar population content of galaxies in the NIR spectral range.

Figures

Figures reproduced from arXiv: 2506.05986 by the authors.

Figure 1
Figure 1. Top panel: K-band radially binned spectra obtained with LUCI@LBT for M87. Each spectrum has a S/N ratio larger than 90 (per Å; see the text). An arbitrary vertical shift has been applied among different spectra. The main absorption features (see Sect. 4) are marked with gray shaded regions. Different colors correspond to different galactocentric distances, with red marking the center and blue the outermost radial bi… view at source ↗
Figure 2
Figure 2. K-band spectra of M87 in the regions where spectral indices are measured. From left to right, and top to bottom, the plots correspond to CaI2.26 and NaI2.21 (first row), FeI2.23 and FeI2.24 (second row), MgI2.10 and MgI2.28 (third row), CO2.30 and CO2.32 (bottom row), respectively. Spectra with different colors correspond to different galac￾tocentric distances, with the same color coding as in [PITH_FULL_IMAGE:figu… view at source ↗
Figure 3
Figure 3. K-band line-strength indices as a function of normalized galac￾tocentric distance, R/Re. From left to right, and top to bottom, the Fig￾ure plots the same indices as in [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Same as [PITH_FULL_IMAGE:figures/full_fig_p008_4.png]

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