Tracing nitrogen enrichment across cosmic time with JWST

¹ Università di Firenze, Dipartimento di Fisica e Astronomia, Via G. Sansone 1, 50019 Sesto Fiorentino, Florence, Italy
² INAF – Arcetri Astrophysical Observatory, Largo E. Fermi 5, I-50125 Florence, Italy
³ European Southern Observatory, Karl-Schwarzschild Straße 2, D-85748 Garching bei München, Germany
⁴ School of Astronomy, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531 Tehran, Iran
⁵ Department of Physics, University of Arkansas, 226 Physics Building, 825 West Dickson Street, Fayetteville, AR 72701, USA
⁶ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁷ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
⁸ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ University of Trento, Via Sommarive 14, I-38123 Trento, Italy

^★ Corresponding author.

Received: 4 December 2025
Accepted: 25 March 2026

Abstract

We present a comprehensive analysis of the nitrogen-to-oxygen (N/O) abundance ratio in a sample of ∼660 star-forming galaxies at redshift z ∼ 1 − 6, with a median redshift of ⟨z⟩ = 3.0, using deep JWST/NIRSpec spectroscopy. Leveraging detections of faint auroral emission lines in 92 galaxies at z > 1 from both the MARTA survey and a large compilation of high-redshift literature objects, we derived 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 a mild evolution for the N2O2 = log([N II] λ6585/[O II] λλ3727,3729) diagnostic and no clear evolution for the N2S2 = log([N II] λ6585/[S II] λλ6717,6731) diagnostic compared to local realisations. We then investigated the N/O–O/H relation across cosmic time using both ‘direct’ abundances and strong-line-based measurements (additional 535 galaxies). We find evidence of 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 local trends, with a more pronounced enhancement at low metallicity (i.e. 12 + log(O/H)≲8.1), where the offset reaches up to ∼0.4−0.5 dex. We considered several scenarios to explain the observed trends, including bursty star formation, differential metal loading, and inflows of pristine gas. Our results provide the most extensive confirmation of elevated N/O ratios at high-redshift to date based on rest-optical diagnostics and within a self-consistent frame.