Astrochemistry

Tracing Nitrogen Enrichment Across Cosmic Time With JWST

By Keith Cowing
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astro-ph.GA
December 22, 2025
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Tracing Nitrogen Enrichment Across Cosmic Time With JWST
Relation between N/O and O/H for our full sample, as described in Section 2. Gold hexagons indicate MARTA galaxies with direct measurements, and light-blue triangles show the JWST auroral-line dataset. High-redshift strong-line galaxies from the literature are plotted as teal triangles, and local galaxies (both direct and strong-line measurements) as grey circles. White squares represent local galaxies binned in metallicity, showing the median N/O value in each bin. Two violet stars indicate the median N/O values for high-redshift galaxies, with the first corresponding to 12 + log(O/H) < 8.1 and the second to 12 + log(O/H) ≥ 8.1. The black line shows the best-fit relation derived from the binned values of local galaxies using the functional form from HaydenPawson et al. (2022), with the best-fit parameters obtained in this work and reported in Table 1. The solar abundance values from Asplund et al. (2021) are shown with the orange Sun symbol. Finally, the inset panel presents the histogram of N/O residuals relative to the low-redshift fit, separately for local and high-redshift galaxies, with vertical dashed lines marking the medians of the two distributions. -- astro-ph.GA

We present a comprehensive analysis of the nitrogen-to-oxygen (N/O) abundance ratio in star-forming galaxies at redshift z~1-6, with a median redshift of z=2.7, using deep JWST/NIRSpec spectroscopy.

Leveraging detections of faint auroral emission lines in 76 galaxies at z>1 from both the MARTA survey and a large compilation of high-redshift literature objects, we derive direct electron temperature-based abundances for nitrogen and oxygen using rest-frame optical lines. We establish the first high-redshift calibrations of strong-line N/O diagnostics based on direct abundance measurements, finding no significant evolution for either N2O2 = [NII]6585/[OII]3727,3729 and N2S2 = [NII]6585/[SII]6717,6731 diagnostics compared to local realisations.

We then investigate the N/O-O/H relation across cosmic time using both direct abundances and strong-line based measurements (additional 430 galaxies). We find evidence for mild but systematic nitrogen enhancement at high redshift: galaxies at z>1 exhibit N/O ratios elevated by ~0.18 dex (median offset) at fixed O/H compared to the local relation, with a more pronounced enhancement at low metallicity (12+log(O/H) < 8.1) where the offset reaches up to ~0.3-0.4 dex.

Our results provide the most extensive confirmation of elevated N/O ratios at high-redshift to date based on rest-optical diagnostics. The chemical signatures of N/O-enhanced galaxies in our sample resembles that of first-generation globular cluster stars, suggesting that the moderate nitrogen enhancement may reflect the late stages of a cluster-driven enrichment mode that dominated at earlier cosmic epochs.

However, the relevance and relative contribution of different mechanisms (e.g. burstiness of the star-formation history, contribution from older stellar populations, differential metal-loaded outflows, inflows of pristine gas) remains to be fully disentangled.

E. Cataldi, F. Belfiore, M. Curti, B. Moreschini, A. Marconi, R. Maiolino, A. Feltre, M. Ginolfi, F. Mannucci, G. Cresci, X. Ji, A. Amiri, M. Arnaboldi, E. Bertola, C. Bracci, M. Ceci, A. Chakraborty, F. Cullen, Q. D’Amato, C. Kobayashi, I. Lamperti, C. Marconcini, M. Scialpi, L. Ulivi, M. V. Zanchettin

Comments: 16 pages, 11 figures, submitted to A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:2512.07955 [astro-ph.GA] (or arXiv:2512.07955v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2512.07955
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Submission history
From: Elisa Cataldi
[v1] Mon, 8 Dec 2025 19:00:06 UTC (4,592 KB)
https://arxiv.org/abs/2512.07955

Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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