The main thing to know is that this work uses new JWST/NIRSpec medium-resolution spectra for 14 massive quiescent galaxies at 3
Referee Report
0 major / 2 minor
Summary. The paper presents deep JWST/NIRSpec spectroscopy for 14 massive quiescent galaxies at 3<z<5 from the EXCELS survey. After custom re-reduction to mitigate spectral wiggles from undersampling, full-spectral fitting is used to derive star-formation histories and metallicities, revealing a clear stellar age–stellar mass correlation in which more massive systems assembled earlier. This is interpreted as spectroscopic confirmation that archaeological downsizing was already in place by z≃4, with a slope (~2 Gyr per dex) matching lower-redshift literature; no old low-mass quiescent objects are found, and high metallicities are reported, while α-enhancement measurements show model-dependent discrepancies.
Significance. If the reported age–mass correlation holds, the result supplies direct spectroscopic evidence for the early onset of downsizing, extending the trend to z≃4 and aligning with lower-redshift archaeological studies. The explicit flagging of model dependencies for abundances (rather than ages) and the absence of old low-mass systems are useful constraints on high-redshift galaxy assembly.
minor comments (2)
- [Results] The slope of the age–mass relation is stated as ≃2 Gyr per dex; reporting the formal fit value with uncertainty and the exact mass range used would allow direct comparison to literature relations.
- [Chemical abundances discussion] The conclusion that detailed chemical abundances remain challenging is well-supported by the reported model disagreements, but a short table summarizing the range of [α/Fe] values across codes, wavelength ranges, and SFH assumptions would improve clarity.
Simulated Author's Rebuttal
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unresolved
We thank the referee for their positive and accurate summary of our manuscript, as well as for recommending acceptance. The review correctly captures our spectroscopic confirmation of the age-mass correlation in 3<z<5 quiescent galaxies, the consistency of the downsizing slope with lower-redshift studies, the absence of old low-mass systems, the high metallicities, and the model-dependent challenges in measuring alpha-enhancements.
Circularity Check
0 steps flagged
No circularity: observational correlation from new spectra
full rationale
The paper measures stellar ages and masses via full-spectral fitting applied to newly acquired JWST/NIRSpec spectra of 14 galaxies. The reported age-mass correlation is a direct empirical result from these fits on external data; no equation or procedure defines one fitted quantity in terms of another to force the trend, and no self-citation supplies the central claim. Model dependencies are explicitly discussed for abundances but do not propagate into the age recovery that drives the headline result. The derivation chain is therefore self-contained against the input spectra and standard SPS models.
Axiom & Free-Parameter Ledger
1 free parameters ·
2 axioms ·
0 invented entities
The analysis rests on standard stellar population synthesis models and assumptions about star-formation history parametrization; no new entities are introduced.
free parameters (1)
- star-formation history parameters
Ages, metallicities and SFH parameters are fitted to the spectra; these are the direct outputs of the modeling.
axioms (2)
- domain assumption Stellar population synthesis models accurately reproduce the observed spectra of quiescent galaxies
Invoked throughout the full-spectral fitting section; the paper notes disagreements between models for abundances.
- standard math Standard flat Lambda-CDM cosmology for converting redshifts to ages
Used implicitly when reporting ages in Gyr.
pith-pipeline@v0.9.0 ·
5803 in / 1413 out tokens ·
50216 ms ·
2026-05-16T06:38:05.645267+00:00
· methodology
read the original abstract
We present deep, medium-resolution $\lambda=1-5\,\mu$m JWST/NIRSpec spectroscopy for 14 quiescent galaxies at $3<z<5$ with $\log_{10}(M_*/\mathrm{M_\odot}){\,>\,}10$, obtained as part of the EXCELS survey. We perform a complete re-reduction of these data, including a custom optimal-extraction approach to combat the spectral "wiggles" that result from undersampling of the NIRSpec spatial PSF. We constrain the star-formation histories and stellar metallicities of these objects via full-spectral fitting, finding a clear stellar age vs stellar mass correlation, in which more massive galaxies assembled their stellar mass at earlier times. This confirms spectroscopically that the archaeological "downsizing" trend was already in place by $z\simeq4$. The slope of our measured relation ($\simeq2$ Gyr per dex in stellar mass) is consistent with literature results at $0 < z < 3$. We do not observe objects with $\log_{10}(M_*/\mathrm{M_\odot})\lesssim10.5$ and ages of more than a few hundred Myr at this epoch, suggesting that recently reported examples of higher-redshift quiescent galaxies at these masses are likely to soon rejuvenate. We measure relatively high stellar metallicities for the majority of our sample, consistent with similar objects at $0 < z < 3$. Finally, we explore evidence for $\alpha$-enhancement in six older and more luminous galaxies within our sample, finding considerable disagreements in the chemical abundances measured using different stellar population models, different fitted rest-frame wavelength ranges, star-formation history models and fitting codes. We therefore conclude that inferring detailed stellar chemical abundances for the earliest quiescent galaxies remains challenging, and higher signal-to-noise spectra are required (SNR per resolution element $>100$ for $R\simeq1000$).