CECILIA: A direct-method metallicity calibration for Cosmic Noon through the Epoch of Reionization

Tracing the build-up of metals over cosmic time is a key science goal of JWST, but the primary diagnostics of chemical abundances are indirect and currently without suitable calibrations at high redshifts. Various "strong line" metallicity indicators that work well in the local universe produce conflicting predictions at z~2-3 because they rely on intrinsic correlations among the physical conditions in galaxies' star-forming regions and their massive stars, both of which are significantly different in the early universe. This program uses NIRSPec to provide a new calibration sample of galaxy spectra to characterize these intrinsic correlations in situ at z=2. The proposed observations will enable "direct" electron-temperature-based measurements of gas-phase metallicities as well as independent estimates of the hardness of the stellar ionizing fields. In turn, these data will enable the re-calibration of strong-line metallicity estimators and improve the interpretation of photoionization models at high redshift. In contrast to other methods of calibrating these relationships, the [SIII]6312+[OII]7320,30 method proposed here is efficient over a broad parameter space of galaxy properties, including at high metallicities where current methods disagree, and it is uniquely achievable using JWST. The proposed sample includes galaxies selected using multiple methods over a wide range in stellar mass to maximize the applicability of the resulting relations to diverse galaxies across cosmic time. The outcome of this proposal will be tools that can be used to determine chemical abundances for thousands of future objects observed by JWST from z=2 to the Epoch of Reionization at z>6.