The JWST excels survey: gas-phase metallicity evolution at 2 < z < 8

Abstract

We present an analysis of the gas-phase mass–metallicity relationship (MZR) and fundamental metallicity relationship (FMR) for 65 star-forming galaxies at (Formula presented) from the JWST/EXCELS survey. We calculate gas-phase metallicities (12 + log(O/H)) using strong-line calibrations explicitly tested against the EXCELS sample, and report direct-method metallicities for 19 galaxies. Our sample spans (Formula presented) and (Formula presented), consistent with main-sequence star-forming galaxies at the same redshifts. We find a clear MZR at both (Formula presented) ((Formula presented)) and (Formula presented) ((Formula presented)), with consistent slopes and mild evolution in normalization of (Formula presented), matching trends from simulations and recent observations. Our results demonstrate rapid gas-phase enrichment in the early Universe; galaxies at (Formula presented) (within the first (Formula presented) per cent of cosmic time) are enriched to (Formula presented) per cent of the metallicity of equivalent mass galaxies at (Formula presented). We find tentative evidence for SFR-dependence in the MZR scatter, though results remain inconclusive and highlight the need for larger high-redshift samples. Comparison with locally derived FMRs reveals a clear offset consistent with other (Formula presented) studies. We discuss potential drivers of this offset, noting that high-redshift samples have significantly different physical properties compared to local samples used to define the (Formula presented) FMR. Our results confirm that low-mass, high specific star-formation rate galaxies common at high redshift are inconsistent with the equilibrium conditions underlying the local FMR, and highlight the rapid chemical enrichment at early cosmic epochs.