The AURORA Survey: The Mass–Metallicity and Fundamental Metallicity Relations at z ∼ 2.3 Based Purely on Direct T e Metallicities

Abstract We present new constraints on the mass–metallicity (MZR) and fundamental metallicity relations (FMR) using a sample of 34 galaxies at 1.38 ≤ z ≤ 3.5 (median z = 2.28). These galaxies have direct T e measurements from [O iii ]4363 Å and/or [O ii ]7320,7331 Å auroral emission lines detected with JWST/NIRSpec as part of the AURORA survey. The detection of both oxygen auroral lines allows for dual-zone direct T e measurements and expands the dynamic range in 12 + log ( O / H ) (7.68–8.65 dex), stellar mass (10 8 –10 10.4 M ⊙ ), and star formation rate (1–100 M ⊙ yr −1 ) compared to previous direct T e studies of the high-redshift MZR and FMR. We characterize the z ∼ 2 MZR and find a slope of 0.27 ± 0.04 and normalization of 12 + log ( O / H ) = 8.44 ± 0.04 at10 10 M ⊙ with an intrinsic scatter of 0.10 dex, consistent with past strong-line MZR measurements. Comparisons with z ∼ 2 predictions from six simulations reveal that none reproduce our observed MZR normalization evolution between z ∼ 0 and z ∼ 2. This discrepancy suggests current models do not fully capture the chemical enrichment and feedback processes occurring at cosmic noon. However, all 34 galaxies are on or above the star-forming main sequence such that our sample may be biased toward lower 12 + log ( O / H ) if the FMR persists at z ∼ 2. Correcting for this selection effect would increase O/H by ≈0.1 dex at 10 9.3 M ⊙ (the median mass of our sample), bringing our MZR into better agreement with that of Illustris TNG . Lastly, we find our z ∼ 2.3 sample is consistent with the z ∼ 0 FMR within 0.1 dex in O/H, indicating that the smooth secular mechanisms regulating chemical enrichment, star formation, stellar mass, and outflows were in place at cosmic noon.