The HyLight model for hydrogen emission lines in simulated nebulae

Hydrogen recombination lines provide key diagnostics of ionized gas in galaxies, yet most hydrodynamical simulations estimate hydrogen level populations using interpolated emissivity tables rather than computing them directly from local physical conditions. We present HyLight, a Python-based atomic model that calculates hydrogen level populations and line emissivities from the gas density, temperature, and ionization state, enabling accurate predictions in both equilibrium and non-equilibrium environments. Benchmark comparisons show that HyLight reproduces Cloudy predictions for Balmer, Paschen, and Brackett emissivities to within 1 per cent under typical photoionized nebular conditions, while discrepancies of several tens of per cent arise relative to other published calculations. As an illustrative application, we use HyLight to compute photoionization-to-line intensity ratios in an HII nebula and generate synthetic hydrogen emission maps from a radiation-hydrodynamical simulation that includes non-equilibrium thermochemistry. Combining physical consistency with flexibility, HyLight provides a robust framework for connecting hydrodynamical simulations with observational diagnostics of photoionized regions, and enhances our ability to interpret hydrogen emission in complex, non-equilibrium astrophysical environments.