The First Empirical Calibration of the Mid-infrared Abundance Diagnostic Ne₂₃ with JWST

Large surveys of galaxies in the local and high-redshift Universe have, traditionally, relied on the intensity of rest-optical emission lines from metal ions in the interstellar medium (ISM) to indirectly estimate the O/H abundance in the gas. However, these optical strong line diagnostics are also sensitive to the electron gas temperature (T_e), resulting in large systematic uncertainties that inherently limit their utility as metallicity tracers, especially in dust-obscured and metal-rich environments. To this end, we provide the first empirical calibration of Ne₂₃, a novel abundance diagnostic using the mid-infrared (MIR) T_e-insensitive [Ne ii]λ12.81 μm and [Ne iii]λ15.56 μm fine-structure lines. We present new JWST/MIRI medium-resolution spectroscopy (MRS) observations of 10 H ii regions with optical measurements of T_e and O/H from the CHAOS project, and we analyze MIRI observations of eight low-metallicity galaxies with similarly high-fidelity direct O/H. We measure Ne₂₃ from 1D MIR spectra extracted from apertures matched to the ground-based spectroscopy used to obtain O/H, a method that is unfeasible from MIR spectra acquired on prior space-based observatories. From these nebulae, Ne₂₃ is strongly correlated with O/H over 1.5 dex in 12+log(O/H). We calibrate the O/H–Ne₂₃ relation from the empirical data, finding a scatter of just 0.06 dex in O/H at fixed Ne₂₃. The O/H–Ne₂₃ relation presented here provides a means to reliably estimate 12+log(O/H) from JWST/MIRI MRS observations of ionized nebulae out to z ≈ 0.8, enabling new chemical abundance surveys of highly attenuated regions and in the metal-rich ISM.