A Self-Consistent Direct Method for Chemical Abundances in High- z Galaxies with JWST

Abstract The unprecedented rest-frame UV and optical coverage provided by JWST enables simultaneous constraints on the electron density (ne) and temperature (Te) of ionized gas in galaxies at z 5. We present a self-consistent direct method based on multiple O iii] (λ1661,66) and O iii (λ4363, and λ5007) transitions to characterize the physical conditions of the high-ionization zone. This new approach is insensitive to a wide range of ne due to the high critical densities of the O iii] and O iii transitions. Applying this technique to six galaxies at z ~ 5-9, we find electron densities up to ne ~3 × 105 cm−3 and temperatures of Te ~20, 000 K in systems at z 6. Accounting for these self-consistent densities changes the derived Te and modifies the inferred metallicities by up to 0.29 dex relative to previous estimates. We discuss the reported N/O overabundances in our high-z sample, driven by the high N3 +/H+ ratios inferred from the N iv] lines. We point out that a Te-stratification, in which the N3 + zone has a slightly higher Te than Te(O iii), could substantially reduce the inferred N/O. Quantitatively, if Te(N3 +) were 10% higher than Te(O iii), this could induce a systematic overestimation of N3 +/O2 + of nearly 50%. Classical N/O diagnostics such as N+/O+, due to their critical densities, can significantly impact the inferred N/O abundance in the presence of high-density gas, whereas N2 +/O2 + place these galaxies closer to z ~ 0 systems in the N/O-O/H plane. While promising, this method accounts for density inhomogeneities but not Te fluctuations, and relies on UV atomic data in the high-ne regime. Future JWST programs will be key to testing its robustness.