Theoretical emission lines and metallicity calibrations of H II regions in ASTRID simulation

Abstract We present a theoretical framework to derive redshift-dependent metallicity calibrations for galaxies at z=2-7. The ionization parameter (U) and gas pressure (P) in our approach are not assumed, but are predicted self-consistently. By combining the ASTRID cosmological simulation with stellar population synthesis (SPS) and MAPPINGS V photoionization modeling, we evolve young star clusters under an analytic wind-driven bubble model. This directly couples stellar feedback to the local ISM density, allowing H ii region properties to emerge from the underlying physics rather than being treated as free parameters. The emission-line predictions are validated against observed star-formation rate indicators (deviation <0.05 dex) and the [O iii] luminosity function. We derive calibrations for common optical (e.g. R23, O3N2, N2, O32) and UV (e.g. C3O3, N3O3) diagnostics. We find significant redshift evolution in these relations, driven primarily by changing ionization conditions. A Bayesian analysis quantifies calibration performance under varying signal-to-noise, enabling diagnostic recommendations as a function of redshift and data quality. The R23 calibration performs well at all redshifts with minimal error in our model, while nitrogen- and carbon-based calibrations are highly sensitive to the abundance enrichment process and should be used with caution. These results provide a practical framework for interpreting JWST spectroscopy and tracing chemical evolution from cosmic noon to the epoch of reionization.