The JWST EXCELS Survey: gas-phase metallicity evolution at 2 < z < 8

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 $8.1<\log ({\mathrm{M}}_{\star }\mathrm{/}{\mathrm{M}}_{\odot })<10.3$ and $0<\log (\mathrm{S}\mathrm{F}\mathrm{R}\mathrm{/}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1})<2$, 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 $\simeq 0.1\mathrm{d}\mathrm{e}\mathrm{x}$, matching trends from simulations and recent observations. Our results demonstrate rapid gas-phase enrichment in the early Universe; galaxies at $z\simeq 3$ (within the first $\simeq 15$ per cent of cosmic time) are enriched to $\simeq 40$ 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.