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Arxiv: The JWST EXCELS survey: Probing strong-line diagnostics and the chemical evolution of galaxies over cosmic time using Te-metallicities Published: 2/14/2025 9:00:00 PM Updated: 2/14/2025 9:00:00 PM

Paper abstract: We present an analysis of the rest-frame optical spectra of 22[OIII]\lambda4363 detected galaxies in the redshift range 1.65 < z < 7.92(with \langle z \rangle = 4.05) from JWST/NIRSpec medium-resolutionobservations taken as part of the EXCELS survey. To supplement thesehigh-redshift sources, we also consider a sample of 782 local[OIII]\lambda4363 detected galaxies from the DESI Early Data Release. Ouranalysis demonstrates that many strong-line calibrations are biased in theearly Universe due to the systematic evolution in ionization conditions withredshift. However, the recently introduced \widehat{R} calibration mostlyremoves the dependence on ionization state and can be considered a largelyredshift-independent calibration. In a similar spirit, we introduce a newstrong-line diagnostic, \widehat{RNe}, which can be used to robustly estimatemetallicities when the [OIII]\lambda5007 is redshifted out of the wavelengthrange of JWST/NIRSpec at z > 9.5. We also show that strong-line diagnosticsusing the [NII]\lambda6584 emission line are likely to be biased athigh-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[OIII]\lambda4363 detected galaxies at z ~eq 4 on the mass-metallicityplane and investigate the redshift evolution of the fundamental metallicityrelation (FMR). We find tentative evidence for an increasing deviation from theFMR at z > 4 which might indicate fundamental differences in the baryon cycleat these redshifts. However, more data are required as our high-redshiftconstraints are still based on a relatively small sample of galaxies and thesignificance of the deviation is strongly dependent on the assumed form of thefundamental metallicity relation.