Novel z ∼ 10 auroral line measurements extend the gradual offset of the fundamental metallicity relation deep into the first gigayear of cosmic time

¹ Cosmic Dawn Center (DAWN), Denmark
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
³ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Department of Physics & Astronomy, University College London, London WC1E 6BT, UK
⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁶ Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA, Leiden, The Netherlands
⁷ DTU Space, Technical University of Denmark, Elektrovej 327, DK2800 Kgs. Lyngby, Denmark
⁸ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria

^★ Corresponding author.

Received: 19 June 2025
Accepted: 30 January 2026

Abstract

The mass assembly and chemical enrichment of the first galaxies provide key insights into their star formation histories and the earliest stellar populations at cosmic dawn. Here we compile and utilise new, high-quality spectroscopic JWST/NIRSpec Prism observations from the JWST archive. In particular, we extend the wavelength coverage beyond the standard pipeline cut-off (5.3 μm) up to 5.5 μm, which enables for the first time a detailed examination of the rest-frame optical emission-line properties for galaxies at z ≈ 10. Crucially, the improved calibration allows us to detect Hβ and the [O III] λλ4959, 5007 doublet and resolve the auroral [O III] λ4363 line for the 11 galaxies in our sample (z = 9.3 − 10.0) to obtain direct T_e-based metallicity measurements. We find that the interstellar medium (ISM) of all galaxies shows high ionisation fields and electron temperatures, with derived metallicities in the range 12 + log(O/H) = 7.1 − 8.3 (3–50% solar), consistent with previous strong-line diagnostics based on JWST data at high redshifts. We derive an empirical relation for M_UV and 12 + log(O/H) at z ≈ 10, useful for future higher-redshift studies, and show that the sample galaxies are ‘typical’ star-forming galaxies though with relatively high specific star formation rates (median sSFR = SFR_Hβ/M_★ = 38 Gyr⁻¹) and with evidence of bursty star formation on 10 Myr versus 100 Myr timescales (log₁₀(SFR₁₀/SFR₁₀₀)≈0.7). Combining the rest-frame optical line analysis and detailed UV to optical spectro-photometric modelling, we determine the mass-metallicity relation (MZR) and the fundamental metallicity relation (FMR) of the sample, pushing the previous redshift frontier of these measurements to z = 10. These results, together with literature measurements, point to a gradually decreasing MZR at higher redshifts, with a break in the FMR at z ≈ 3, decreasing to metallicities ≈3× lower at z = 10 than observed in galaxies during the majority of cosmic time at z = 0 − 3, likely caused by massive pristine gas inflows diluting the observed metal abundances during early galaxy assembly at cosmic dawn.