Rapid, out-of-equilibrium metal enrichment indicated by a flat mass-metallicity relation at z  ∼  6 from NIRCam grism spectroscopy

¹ Institute of Science and Technology Austria Am Campus 1 3400 Klosterneuburg, Austria
² National Astronomical Observatory of Japan 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
³ Astronomy Centre, University of Sussex, Falmer Brighton BN1 9QH, UK
⁴ Department of Physics, North Carolina State University Raleigh 27695 North Carolina, USA
⁵ Institute for Computational & Data Sciences, The Pennsylvania State University University Park PA 16802, USA
⁶ Institute for Gravitation and the Cosmos, The Pennsylvania State University University Park PA 16802, USA
⁷ Kavli Institute for Cosmology, University of Cambridge Madingley Road Cambridge CB3 0HA, UK
⁸ Cavendish Laboratory, University of Cambridge JJ Thomson Avenue Cambridge CB3 0HE, UK
⁹ Department of Astronomy, University of Wisconsin-Madison 475 N. Charter St. Madison WI 53706, USA
¹⁰ Centro de Astrobiología (CAB), CSIC-INTA Carretera de Ajalvir km 4 Torrejón de Ardoz E-28850 Madrid, Spain
¹¹ Cosmic Dawn Center (DAWN), Denmark
¹² Niels Bohr Institute, University of Copenhagen Jagtvej 128 2200 Copenhagen N, Denmark
¹³ Department of Astronomy, University of Geneva Chemin Pegasi 51 1290 Versoix, Switzerland
¹⁴ Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny 1290 Versoix, Switzerland
¹⁵ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA
¹⁶ BNP Paribas Corporate & Institutional Banking Torre Ocidente Rua Galileu Galilei 1500-392 Lisbon, Portugal

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Received: 25 July 2025
Accepted: 25 November 2025

Abstract

We aim to characterise the mass-metallicity relation (MZR) and the 3D correlation between the stellar mass, metallicity, and star formation rate (SFR) known as the fundamental metallicity relation (FMR) for galaxies at 5 < z < 7. Using ∼800 [O III] selected galaxies from deep NIRCam grism surveys, we present our stacked measurements of direct-T_e metallicities, which we used to test recent strong-line metallicity calibrations. Our measured direct-T_e metallicities (0.1–0.2 Z_⊙ for M_★ ≈ 5 × 10^7 − 9 M_⊙, respectively) match recent JWST/NIRSpec-based results. However, there are significant inconsistencies between observations and hydrodynamical simulations. We observe a flatter MZR slope than the SPHINX²⁰ and FLARES simulations, which cannot be attributed to selection effects. With simple models, we show that the effect of an [O III] flux-limited sample on the observed shape of the MZR is strongly dependent on the FMR. If the FMR is similar to the one in the local Universe, the intrinsic high-redshift MZR should be even flatter than is observed. In turn, a 3D relation where SFR correlates positively with metallicity at fixed mass would imply an intrinsically steeper MZR. Our measurements indicate that metallicity variations at fixed mass show little dependence on the SFR, suggesting a flat intrinsic MZR. This could indicate that the low-mass galaxies at these redshifts are out of equilibrium and that metal enrichment occurs rapidly in low-mass galaxies. However, being limited by our stacking analysis, we are yet to probe the scatter in the MZR and its dependence on SFR. Large carefully selected samples of galaxies with robust metallicity measurements can put tight constraints on the high-redshift FMR and help us to understand the interplay between gas flows, star formation, and feedback in early galaxies.