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 $2 \lt z \lt 8$ 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 $8.1\lt \log (\rm \mathit{ M}_\star /M_\odot)\lt 10.3$ and $0\lt \log (\rm SFR/M_\odot \, yr^{-1})\lt 2$, consistent with main-sequence star-forming galaxies at the same redshifts. We find a clear MZR at both $2\lt z\lt 4$ ($\langle z \rangle = 3.2$) and $4\lt z\lt 8$ ($\langle z \rangle = 5.5$), with consistent slopes and mild evolution in normalization of ${\simeq} 0.1 \, \mathrm{dex}$, 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 $z \gt 3$ 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.