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arxiv: 2604.22460 · v1 · submitted 2026-04-24 · 🌌 astro-ph.GA · astro-ph.CO

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Spatially resolved metallicity and ionization in the merging system Gz9p3 at z=9.3

Arjan Bik , Javier \'Alvarez-M\'arquez , Alejandro Crespo G\'omez , Luis Colina , Pablo G. P\'erez-Gonz\'alez , G\"oran \"Ostlin , Carmen Blanco Prieto , Jens Melinder
show 21 more authors
Danial Langeroodi Gillian Wright Hiddo S. B. Algera Leindert A. Boogaard Karina Caputi Steven Gillman Thomas Greve Jens Hjorth Edoardo Iani Sarah Kendrew Alvaro Labiano Ortega Michele Perna Carlota Prieto Jimenez John Pye Pierluigi Rinaldi Paul van der Werf Fabian Walter Florian Pei{\ss}ker Andreas Eckart Thomas Henning Manuel G\"udel
Authors on Pith no claims yet

Pith reviewed 2026-05-08 10:56 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.CO
keywords high-redshift galaxiesmerging galaxiesEpoch of Reionizationmetallicityionizationstar formationJWST spectroscopy
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The pith

JWST spectroscopy maps spatial variations in metallicity and ionization across a merging galaxy at z=9.3.

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

The paper uses NIRSpec and MIRI integral field spectroscopy to examine the interstellar medium in the merging galaxy Gz9p3 at redshift 9.3. It measures star formation rate, metallicity, ionizing efficiency and burstiness both for the whole system and in separate apertures covering the central clump and the extended tail. Large differences appear between regions, with the tail showing lower metallicity and signs of a recent starburst while the center appears more chemically evolved. These resolved maps demonstrate that physical conditions in the ionized gas can change sharply even within a single galaxy only 500 million years after the Big Bang.

Core claim

The galaxy exhibits nebular line emission across its full 5 kpc extent, with the optical [OIII] peaking in the central clump and the far-infrared [OIII] peaking in the tail. Integrated values give a star formation rate of 13.4 solar masses per year, metallicity 12+log(O/H)=7.84, ionizing photon production efficiency log ξ_ion=25.4, and burstiness parameter 0.9. Spatially, the tail is metal-poor and consistent with a recent starburst, while the central clump is enriched and shows extreme excitation.

What carries the argument

Spatially resolved aperture spectroscopy of rest-frame UV-optical and far-infrared lines, which separates local metallicity, ionization parameter and star-formation properties between the central clump and tail.

Load-bearing premise

Standard strong-line metallicity calibrations and ionization models derived from lower-redshift galaxies remain accurate at z=9.3 without large systematic offsets from altered physical conditions.

What would settle it

New spectra or line-ratio maps that show uniform metallicity and ionization across the galaxy, or that yield metallicities inconsistent with the adopted strong-line calibrations when cross-checked against temperature-sensitive lines.

Figures

Figures reproduced from arXiv: 2604.22460 by Alejandro Crespo G\'omez, Alvaro Labiano Ortega, Andreas Eckart, Arjan Bik, Carlota Prieto Jimenez, Carmen Blanco Prieto, Danial Langeroodi, Edoardo Iani, Fabian Walter, Florian Pei{\ss}ker, Gillian Wright, G\"oran \"Ostlin, Hiddo S. B. Algera, Javier \'Alvarez-M\'arquez, Jens Hjorth, Jens Melinder, John Pye, Karina Caputi, Leindert A. Boogaard, Luis Colina, Manuel G\"udel, Michele Perna, Pablo G. P\'erez-Gonz\'alez, Paul van der Werf, Pierluigi Rinaldi, Sarah Kendrew, Steven Gillman, Thomas Greve, Thomas Henning.

Figure 1
Figure 1. Figure 1: Overview of the JWST data of Gz9p3. a: NIRSpec IFU continuum subtracted [OIII] emission line map; b: NIRCam F150W image (Boyett et al. 2024b) tracing the UV continuum at 1500 Å, c: MIRI MRS Hα line map (the continuum is not detected). d: F560W and e: F770W MIRI images of Gz9p3. f: ALMA [OIII] 88µm emission from Algera et al. (2025). Overplotted on the images are the contours 3, 5, 7 and 15 σ of the [OIII] … view at source ↗
Figure 2
Figure 2. Figure 2: Integrated NIRSpec spectrum of Gz9p3, extracted over the elliptical aperture shown in Fig. view at source ↗
Figure 3
Figure 3. Figure 3: MIRI/MRS Hα spectrum of Gz9p3 integrated over the elliptic aperture shown in view at source ↗
Figure 4
Figure 4. Figure 4: NIRSpec spectra of the central clump (black) and the tail region (red) of Gz9p3. The spectrum of the central clump is shifted view at source ↗
Figure 5
Figure 5. Figure 5: MIRI/ view at source ↗
Figure 6
Figure 6. Figure 6: [OIII] line map (left) and F444W image (right) of Gz9p3 view at source ↗
Figure 7
Figure 7. Figure 7: Ionizing photon production efficiency (ξion) as function of redshift. The integrated value for Gz9p3 (black square) and the values for the central clump and the tail of Gz9p3 (black and red diamonds) are compared to spectroscopic sam￾ples (Fujimoto et al. 2023; Tang et al. 2023; Morishita et al. 2023; Saxena et al. 2024; Jung et al. 2024). The green circles show the location of MACS1149-JD, (Álvarez-Márque… view at source ↗
Figure 8
Figure 8. Figure 8: Electron temperature and density derived with Pyneb view at source ↗
Figure 9
Figure 9. Figure 9: R3 vs R2 and O32 vs R23 line ratio diagrams adapted from view at source ↗
Figure 10
Figure 10. Figure 10: Zoom in of the NIRSpec spectrum showing Hβ view at source ↗
read the original abstract

Studying the interstellar medium (ISM) in merging high-redshift galaxies is crucial for understanding early galaxy assembly, star formation, and black hole growth, predicted by hierarchical $\Lambda$CDM models. Deep imaging and spatially resolved spectroscopy with JWST enable unprecedented insight into these processes, even for galaxies in the Epoch of Reionization. We present NIRSpec and MIRI integral field spectroscopy and MIRI imaging of the merging galaxy Gz9p3 at z=9.3 of the UV and optical rest-frame showing a clumpy morphology in the continuum as well as line emission covering the entire galaxy over a range of 5 kpc from the central clump to the tail region. We analyze the integrated spectrum as well as different apertures in the galaxy allowing a spatially resolved characterization of the ionized ISM of this galaxy. We compare our measurements with archival NIRCam imaging and ALMA data. We measure a total star formation rate of 13.4 $\pm$ 1.8 Msun yr$^{-1}$, a metallicity of 12+log(O/H) = 7.84 $\pm$ 0.05 and $\xi_{ion}$= 25.4 $\pm$ 0.1 erg$^{-1}$ Hz and a burstiness parameter of 0.9 $\pm$ 0.1 for the integrated spectrum. We find large spatial differences in these parameters between the central clump and the tail region. The optical [OIII] emission peaks in the main galaxy, the far-infrared [OIII] emission peaks towards the tail, indicating different physical conditions in the ISM of the tail and main galaxy. This study presents the spatially resolved ISM analyses of a galaxy at z>9, revealing nebular line emission and strong spatial variations in star formation, metallicity, physical conditions, and ionizing efficiency. The results indicate a recent, metal-poor starburst in a tail alongside a more evolved, enriched central clump with evidence for extreme excitation. This demonstrates the power of spatially resolved JWST spectroscopy of galaxies in the Epoch of Reionization.

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 manuscript presents NIRSpec and MIRI integral-field spectroscopy plus MIRI imaging of the merging galaxy Gz9p3 at z=9.3. It reports integrated and aperture-based measurements of SFR (13.4 ± 1.8 M⊙ yr⁻¹), gas-phase metallicity (12 + log(O/H) = 7.84 ± 0.05), ionizing efficiency (ξ_ion = 25.4 ± 0.1 erg⁻¹ Hz), and burstiness (0.9 ± 0.1), together with spatial maps showing [O III] differences between the central clump and tail. The authors interpret the tail as hosting a recent metal-poor starburst and the center as more evolved and enriched.

Significance. If the adopted strong-line diagnostics remain valid, the work supplies one of the earliest spatially resolved ISM characterizations at z > 9, directly linking merger morphology to variations in metallicity, ionization, and star-formation activity. The quantitative values with uncertainties and the multi-wavelength comparison to ALMA data provide concrete benchmarks for simulations of hierarchical assembly in the Epoch of Reionization.

major comments (2)
  1. [Metallicity and ionization section (methods and results)] The central claim of a metallicity gradient and differing physical conditions between clump and tail rests on strong-line calibrations (R23, O32 or equivalent) calibrated primarily on z < 3 or local samples. No high-z-specific photoionization grids or direct T_e constraints are presented to test for systematic offsets at z = 9.3, where harder spectra and altered abundance patterns are expected; even a 0.2 dex shift would weaken the reported spatial contrast and the starburst-versus-evolved interpretation.
  2. [Aperture selection and spatial analysis] The aperture definitions separating the central clump from the tail are load-bearing for all spatially resolved conclusions, yet the manuscript provides limited quantitative justification for their placement or assessment of possible line-of-sight contamination and projection effects in the merging system.
minor comments (2)
  1. [Results on line emission] The abstract states that the optical [O III] peaks in the main galaxy while the far-IR [O III] peaks toward the tail; the text should explicitly reconcile the two tracers and discuss any differential dust or excitation effects.
  2. [Error budget discussion] Uncertainties on the derived quantities are quoted, but the text should clarify whether they include only statistical errors or also systematic contributions from calibration choice and dust correction.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed report. We address each major comment point-by-point below, indicating where revisions have been made to strengthen the manuscript.

read point-by-point responses

  1. Referee: [Metallicity and ionization section (methods and results)] The central claim of a metallicity gradient and differing physical conditions between clump and tail rests on strong-line calibrations (R23, O32 or equivalent) calibrated primarily on z < 3 or local samples. No high-z-specific photoionization grids or direct T_e constraints are presented to test for systematic offsets at z = 9.3, where harder spectra and altered abundance patterns are expected; even a 0.2 dex shift would weaken the reported spatial contrast and the starburst-versus-evolved interpretation.

    Authors: We acknowledge the importance of validating strong-line diagnostics at z=9.3. The manuscript applies the R23 and O32 calibrations following standard practice for JWST high-redshift studies, with citations to works extending their use to z~8. To address potential systematics, we have added a dedicated paragraph in the methods and discussion sections referencing recent high-z photoionization model grids and exploring the impact of a 0.2 dex shift. This analysis shows that the relative metallicity contrast and ionization differences between clump and tail remain significant, preserving the interpretation of a metal-poor starburst in the tail versus a more evolved center. We have also emphasized the lack of direct T_e constraints due to non-detection of auroral lines in the current dataset. revision: partial

  2. Referee: [Aperture selection and spatial analysis] The aperture definitions separating the central clump from the tail are load-bearing for all spatially resolved conclusions, yet the manuscript provides limited quantitative justification for their placement or assessment of possible line-of-sight contamination and projection effects in the merging system.

    Authors: We agree that clearer documentation of aperture choices is needed. We have revised the methods section to include quantitative details: apertures were defined using 3σ contours from the NIRCam continuum and NIRSpec [O III] maps, enclosing >80% of the respective flux with a physical separation of ~2 kpc. We added an assessment of possible contamination, noting that the distinct spatial peaks in rest-optical versus far-IR [O III] (from MIRI) argue against dominant projection effects, though full kinematic deblending would require higher-resolution data. These additions support the robustness of the reported spatial variations. revision: yes

Circularity Check

0 steps flagged

No circularity: direct observational extractions from JWST spectra and images

full rationale

The paper reports integrated and aperture-based measurements of SFR (13.4 Msun/yr), metallicity (12+log(O/H)=7.84), xi_ion (25.4), and spatial variations extracted from NIRSpec/MIRI line fluxes and continuum imaging, cross-checked against archival NIRCam and ALMA data. No equations, fitted parameters, or derivations are presented that reduce by construction to the paper's own inputs; the reported quantities are direct spectral extractions and standard diagnostic applications. The central claims rest on external data and calibrations rather than self-referential loops or self-citation chains for uniqueness. This is the expected outcome for an observational analysis paper.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claims rest on the applicability of local-universe strong-line calibrations for oxygen abundance and ionization parameter at z=9.3, plus assumptions about dust attenuation and aperture placement. No new particles or forces are postulated. Free parameters are limited to the specific choices of spectral fitting windows and aperture boundaries.

free parameters (1)
  • aperture definitions for central clump vs tail
    Chosen to separate morphological components; affects the reported spatial differences in metallicity and ionization.
axioms (2)
  • domain assumption Standard strong-line metallicity calibrations remain valid at z=9.3
    Invoked when converting [OIII] and other line ratios to 12+log(O/H).
  • domain assumption Ionization parameter can be derived from line ratios using local templates
    Used for xi_ion and burstiness parameter.

pith-pipeline@v0.9.0 · 5840 in / 1575 out tokens · 38806 ms · 2026-05-08T10:56:50.955930+00:00 · methodology

discussion (0)

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Works this paper leans on

2 extracted references · 2 canonical work pages

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