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ABSTRACT We present an analysis of the rest-frame optical spectra of 22 O iii4363 detected galaxies in the redshift range 1. 65 z 7. 92 (with z = 4. 05) from JWST/NIRSpec medium-resolution observations taken as part of the EXCELS survey. To supplement these high-redshift sources, we also consider a sample of 782 local O iii4363 detected galaxies from the DESI Early Data Release. Our analysis demonstrates that many strong-line calibrations are biased in the early Universe due to the systematic evolution in ionization conditions with redshift. However, the recently introduced R calibration mostly removes the dependence on ionization state and can be considered a largely redshift-independent calibration. In a similar spirit, we introduce a new strong-line diagnostic, RNe (using O ii 3726, 3729, Ne iii3869 and H), which can be used to robustly estimate metallicities when the O iii5007 is redshifted out of the wavelength range of JWST/NIRSpec at z 9. 5. We also show that strong-line diagnostics using the N ii6584 emission line are likely to be biased at high redshift due to a moderate enhancement in the average N/O abundance ratios (at fixed O/H) in these sources. Finally, we discuss the location of our new O iii4363 detected galaxies at z 4 on the mass–metallicity plane and investigate the redshift evolution of the fundamental metallicity relation (FMR). We find tentative evidence for an increasing deviation from the FMR at z 4, which might indicate fundamental differences in the baryon cycle at these redshifts. However, more data are required as our high-redshift constraints are still based on a relatively small sample of galaxies and the significance of the deviation is strongly dependent on the assumed form of the FMR.
Scholte et al. (Wed,) studied this question.