Abstract We present an analysis of the gas-phase mass-metallicity relationship (MZR) and fundamental metallicity relationship (FMR) for 65 star-forming galaxies at 2 z 8 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 8. 1 (M_ /M_) 10. 3 and 0 (SFR/M_ \, yr^-1) 2, consistent with main-sequence star-forming galaxies at the same redshifts. We find a clear MZR at both 2 z 4 (〈z〉 = 3. 2) and 4 z 8 (〈z〉 = 5. 5), with consistent slopes and mild evolution in normalization of ≃ 0. 1 dex, matching trends from simulations and recent observations. Our results demonstrate rapid gas-phase enrichment in the early Universe; galaxies at z ≃ 3 (within the first ≃ 15 per cent of cosmic time) are enriched to ≃ 40 per cent of the metallicity of equivalent mass galaxies at z = 0. 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 z 3 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 z = 0 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.
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