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arxiv: 2606.18244 · v2 · pith:ACPDPHYWnew · submitted 2026-06-16 · 🌌 astro-ph.GA

PAHSPECS: Spatially Resolved PAH Spectroscopy at cosmic noon with JWST MIRI MRS

Fergus R. Donnan , Karin Sandstrom , Irene Shivaei , Leindert A. Boogaard , Tanio D\'iaz-Santos , Cristina M. Lofaro , Rom\'an Fern\'andez Aranda , Manuel Aravena
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Roberto Decarli Hanae Inami Pablo G. P\'erez-Gonz\'alez Gerg\"o Popping Paul P. van der Werf
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Pith reviewed 2026-06-26 23:53 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords PAH spectroscopyJWST MIRIcosmic noonstar-forming galaxiesspatially resolvedISM gradientspolycyclic aromatic hydrocarbonsredshift 1
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The pith

In galaxies at z~1.1, PAHs become larger and more neutral at larger galactocentric radii, the opposite of local galaxy trends.

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

The paper presents spatially resolved JWST MIRI MRS spectroscopy of PAH emission in a representative sample of normal star-forming galaxies at redshift around 1.1. The authors forward-model the data cubes using non-parametric spatial distributions to extract accurate profiles of the 3.3, 6.2, 7.7, and 11.3 micron PAH features after accounting for PSF convolution. From the resulting ratio maps, they report that PAHs increase in size and neutrality with distance from the galaxy center. This radial behavior contrasts with patterns observed in the local universe. The work links the trends to UV radiation field hardness measured from SED fitting and proposes photo-destruction of small ionized PAHs as the driver.

Core claim

Using forward modeling of JWST/MIRI MRS data cubes with non-parametric spatial distributions, the authors extract spatially resolved PAH emission and produce ratio maps showing that PAHs become larger and more neutral with increasing galactocentric radius in z~1.1 star-forming galaxies. This is the opposite of local galaxy trends, indicating different radial ISM gradients at cosmic noon. They also link the trends to UV radiation field hardness via SED fitting, suggesting photo-destruction of small and ionized PAHs.

What carries the argument

Forward modeling of data cubes with non-parametric spatial distributions to recover unbiased PAH spatial profiles after PSF convolution.

If this is right

  • PAH ratio trends with radius indicate different ISM gradients in high-redshift galaxies compared to local ones.
  • The 3.3/11.3 PAH ratio decreases with increasing UV radiation field hardness while 11.3/7.7 increases.
  • Photo-destruction of small and ionized PAHs may drive the radial trends and the overall lower 3.3/11.3 ratio at cosmic noon.
  • PAH properties encode crucial information on the resolved ISM physics in galaxies at cosmic noon.

Where Pith is reading between the lines

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

  • Observations at a range of redshifts could show when the reversal in radial PAH trends occurred.
  • The forward-modeling approach may be applied to other spectral lines to map additional ISM components at high redshift.
  • If confirmed, the result implies that dust grain processing or radiation environments in galaxies change systematically with cosmic time.

Load-bearing premise

The forward-modeling procedure with non-parametric spatial distributions fully recovers unbiased PAH spatial profiles after accounting for PSF convolution, without residual systematics that could mimic or erase the reported radial trends.

What would settle it

Repeating the PAH ratio analysis on similar high-redshift galaxies with an independent extraction method and recovering either flat ratios or the local-universe trend with radius would falsify the claim of reversed gradients.

Figures

Figures reproduced from arXiv: 2606.18244 by Cristina M. Lofaro, Fergus R. Donnan, Gerg\"o Popping, Hanae Inami, Irene Shivaei, Karin Sandstrom, Leindert A. Boogaard, Manuel Aravena, Pablo G. P\'erez-Gonz\'alez, Paul P. van der Werf, Roberto Decarli, Rom\'an Fern\'andez Aranda, Tanio D\'iaz-Santos.

Figure 1
Figure 1. Figure 1: PAH template generated from the average spectrum of 12 star-forming regions in the local LIRGs, NGC 3256 and NGC 7469 (F. R. Donnan et al. 2024a; D. Rigopoulou et al. 2024). The spectrum has been continuum subtracted by fitting with SPIRIT (F. R. Donnan et al. 2024a) and smoothed. The bands highlighted in red show the individual templates used in this work to fit to the PAH￾SPECS data. remove any emission … view at source ↗
Figure 2
Figure 2. Figure 2: Results from cube fitting ASPECS-6 where each row shows the different emission features and the continuum at that wavelength. We show a NIRCAM color image in the top right panel. The first column shows the brightest spaxel with the best fit model feature and continuum. The grey line shows the data smoothed with a Gaussian kernel with a width of 15 wavelength elements. The second column shows the model cont… view at source ↗
Figure 3
Figure 3. Figure 3: Same as [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Same as [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Same as [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Same as [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: PAH ratio maps for the five PAHSPECS galaxies. We only show pixels that have a > 3σ measurement of the ratio. The left column shows NIRCAM color images using the F090W, F200W, F444W filters (M. J. Rieke et al. 2023). The second column shows the PAH6.2/PAH7.7 ratio. The third column shows the PAH11.3/PAH7.7 ratios. The fourth column shows the PAH3.3/PAH11.3 ratios. The final column shows [ArII]/PAH6.2. will… view at source ↗
Figure 8
Figure 8. Figure 8: Plots of various PAH band ratios for the individual spaxels of the five ASPECS galaxies. We show the 11.3/7.7 vs 6.2/7.7 ratios in the first panel and the 11.3/7.7 vs 3.3/11.3 ratios in the second panel and the 6.2/3.3 vs 6.2/7.7 ratios in the third panel. We only show spaxels where there is at least a 3σ measurement for both PAH ratios in each plot. The color of the points indicates the distance from the … view at source ↗
Figure 9
Figure 9. Figure 9: PAH ratios against radius (projected) for each of the galaxies. We plot the 6.2/7.7, 11.3/7.7, 3.3/11.3 and 6.2/3.3 PAH ratios. We show linear fits where there is a > 1σ measurement of the gradient i.e. a linear trend is present. The shaded regions show the 1σ error. We also include a parameter for the intrinsic scatter in x and y, which produces a larger 1σ region than is shaded. PAH molecules by altering… view at source ↗
Figure 10
Figure 10. Figure 10: PAH ratios against the hardness of the radiation field (upper) as measured by the ratio of flux at 207 nm to 550 nm restframe and the specific star-formation rate (lower) for ASPECS-6. We plot the 6.2/7.7, 11.3/7.7, 3.3/11.3 and 6.2/3.3 PAH ratios. We show linear fits where there is a > 1σ measurement of the gradient i.e. a linear trend is present. The shaded regions show the 1σ error. We also include a p… view at source ↗
Figure 11
Figure 11. Figure 11: Demonstration of the effect of different values of the regularisation strength, Γ in equation 3, where the first three panels show the inferred PAH 6.2 maps for ASPECS-6. The first panel shows a low value of Γ where the map is overly noisy while the third panel is over-smoothed with Γ set too high. The second panel is the optimum value of Γ. The final panel shows how the size as measured by the half light… view at source ↗
Figure 12
Figure 12. Figure 12: Testing how the strength of regularisation affects the observed radial trends in the PAH profiles. We show the radial profiles of the various PAH ratios for different strengths of the regularisation via the parameter Γ for the fitting of the 11.3 µm PAH cube. The value of Γ remains fixed at Γ = 1 for the 6.2 µm and 7.7 µm PAH cubes, and Γ = 10 for the 3.3 µm PAH cube. The red box highlights the assumed va… view at source ↗
Figure 13
Figure 13. Figure 13: The measured gradient of the power law fits to the 11.3/7.7 and 3.3/11.3 PAH ratios as a function of the strength of regularisation via the parameter Γ applied to the 11.3 µm PAH fits. The power law fits are shown in [PITH_FULL_IMAGE:figures/full_fig_p021_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Fits to 14 spectra from different regions of M51 using SPIRIT, the spectral decomposition method presented in F. R. Donnan et al. (2024a), to infer PAH fluxes. The PAH ratios are shown as cyan stars in [PITH_FULL_IMAGE:figures/full_fig_p022_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Maps of various galaxy properties extracted through spatially resolved SED fitting of HST and JWST/NIRCAM photometry with BAGPIPES. The left most column shows a three color JWST/NIRCAM image with the subsequent columns showing star-formation rate (SFR), dust corrected UV flux (207 nm), dust corrected optical flux (550 nm), the Av, the hardness of the radiation field as measured by the ratio of UV/Optical … view at source ↗
Figure 16
Figure 16. Figure 16: PAH ratios against the hardness of the radiation field (upper) and the specific star-formation rate (lower) for the remaining four galaxies. We plot the 6.2/7.7, 11.3/7.7, 3.3/11.3 and 6.2/3.3 PAH ratios. We show linear fits where there is a > 1σ measurement of the gradient i.e. a linear trend is present. The shaded regions show the 1σ error [PITH_FULL_IMAGE:figures/full_fig_p023_16.png] view at source ↗
read the original abstract

We present spatially resolved spectroscopy with JWST/MIRI MRS of a representative sample of normal star-forming galaxies at $z\sim1.1$ as part of the PAHSPECS program. To extract emission from Polycyclic Aromatic Hydrocarbon (PAH) features, we forward model the data cubes with non-parametric spatial distributions, accounting for convolution with the PSF. With this method we are able to recover accurate spatial profiles of the 3.3 $\mu$m, 6.2 $\mu$m, 7.7 $\mu$m, 11.3 $\mu$m PAHs and [ArII] (6.98 $\mu$m) emission and produce PAH ratio maps at cosmic noon. From the PAH ratio maps we find that PAHs become larger and more neutral with increasing galactocentric radius, which is the opposite of trends in local galaxies, indicating radial ISM gradients in normal star-forming galaxies are different at cosmic noon. Through spatially resolved SED fitting of HST and JWST photometry we measure the UV radiation field hardness through the intrinsic ratio of UV to optical flux and find the 3.3/11.3 PAH ratio to decrease with increasing hardness and the 11.3/7.7 to increase. This may suggest photo-destruction of small/ionized PAHs is driving the observed PAH ratio trends and may explain the overall lower 3.3/11.3 PAH at cosmic noon compared to the local universe. This work demonstrates that PAH properties hold crucial information on the resolved ISM physics of galaxies at cosmic noon.

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

2 major / 2 minor

Summary. The paper presents JWST/MIRI MRS spatially resolved spectroscopy of PAH features in a sample of normal star-forming galaxies at z~1.1 from the PAHSPECS program. It describes a forward-modeling approach using non-parametric spatial distributions to account for PSF convolution when extracting the 3.3, 6.2, 7.7, and 11.3 μm PAH and [Ar II] emission, producing ratio maps. The central result is that PAH ratios indicate larger and more neutral PAHs with increasing galactocentric radius (opposite to local-universe trends), with supporting SED fitting showing correlations between PAH ratios and UV radiation field hardness that the authors attribute to photo-destruction.

Significance. If the reported radial PAH ratio gradients are shown to be free of residual PSF systematics, the result would indicate that ISM conditions and radial gradients in normal star-forming galaxies at cosmic noon differ from those in the local universe, with implications for PAH processing and galaxy evolution models. The work also demonstrates the utility of resolved mid-IR spectroscopy for high-redshift ISM studies.

major comments (2)
  1. [Abstract] Abstract/Methods (forward-modeling description): The claim that non-parametric spatial distributions recover accurate, unbiased PAH spatial profiles after PSF convolution is load-bearing for all reported radial trends, yet the abstract provides no quantitative validation, simulated-data tests, or error budgets demonstrating that residuals do not bias ratios at the 0.3–1 arcsec scales of the measured gradients.
  2. [Results] Results (PAH ratio maps and radial trends): The headline finding that PAHs become larger and more neutral with radius (opposite local trends) assumes the extracted 3.3/11.3, 11.3/7.7, and 6.2/7.7 maps reflect intrinsic profiles; the finite flexibility of the non-parametric distributions plus wavelength-dependent MIRI MRS PSF and sparse cube sampling can produce correlated residuals that preferentially affect central vs. outer annuli, potentially inverting the sign of the gradient.
minor comments (2)
  1. The abstract refers to 'a representative sample' without stating the number of galaxies, their selection criteria, or redshift distribution.
  2. Notation for the PAH ratios (e.g., exact wavelength ranges or feature definitions used for 3.3/11.3 etc.) should be defined explicitly when first introduced.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which highlight important aspects of our forward-modeling approach and its validation. We address each major comment below and will revise the manuscript to incorporate additional quantitative tests and discussion.

read point-by-point responses

  1. Referee: [Abstract] Abstract/Methods (forward-modeling description): The claim that non-parametric spatial distributions recover accurate, unbiased PAH spatial profiles after PSF convolution is load-bearing for all reported radial trends, yet the abstract provides no quantitative validation, simulated-data tests, or error budgets demonstrating that residuals do not bias ratios at the 0.3–1 arcsec scales of the measured gradients.

    Authors: We agree that the abstract does not include quantitative validation of the non-parametric forward-modeling method. In the revised manuscript we will update the abstract to reference the validation approach and add a dedicated subsection in the Methods section (with supporting figures in an appendix) that presents simulated-data tests. These tests will quantify recovery accuracy, residual levels, and error budgets specifically at the 0.3–1 arcsec scales relevant to the reported gradients. revision: yes

  2. Referee: [Results] Results (PAH ratio maps and radial trends): The headline finding that PAHs become larger and more neutral with radius (opposite local trends) assumes the extracted 3.3/11.3, 11.3/7.7, and 6.2/7.7 maps reflect intrinsic profiles; the finite flexibility of the non-parametric distributions plus wavelength-dependent MIRI MRS PSF and sparse cube sampling can produce correlated residuals that preferentially affect central vs. outer annuli, potentially inverting the sign of the gradient.

    Authors: We acknowledge that wavelength-dependent PSF effects and sparse sampling could in principle introduce correlated residuals capable of biasing central versus outer annuli. The non-parametric distributions are fit independently per wavelength while explicitly including the measured MIRI MRS PSF, which is intended to minimize such biases. Nevertheless, to directly address the possibility of gradient inversion, the revised manuscript will include mock-cube tests with injected radial gradients (both positive and negative) to quantify any residual bias in the recovered PAH ratios and to demonstrate that the observed trends are not artifacts of the method. revision: yes

Circularity Check

0 steps flagged

No circularity: observational extraction of PAH ratio maps from JWST data

full rationale

The paper reports direct observational results from JWST/MIRI MRS spectroscopy of z~1.1 galaxies. PAH spatial profiles are recovered via forward modeling with non-parametric distributions that explicitly account for PSF convolution; the resulting ratio maps (3.3/11.3, 11.3/7.7, 6.2/7.7) and their radial trends are presented as empirical measurements, not as predictions or derivations that reduce to fitted inputs by construction. Spatially resolved SED fitting for UV hardness is likewise an independent photometric analysis. No self-definitional quantities, fitted-input predictions, or load-bearing self-citations appear in the central claims. The analysis chain is self-contained against external data and does not rely on internal tautologies.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review prevents full audit; no explicit free parameters, invented entities, or non-standard axioms are stated.

axioms (1)
  • domain assumption Standard assumptions in PAH feature extraction and SED fitting hold without significant bias at z~1.1
    Invoked implicitly by the forward-modeling and SED methods described in the abstract.

pith-pipeline@v0.9.1-grok · 5893 in / 1293 out tokens · 27900 ms · 2026-06-26T23:53:53.338603+00:00 · methodology

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