Predicting ionized gas emission in 3D with SKIRT. I. Framework and validation

Emission lines from ionized gas are key diagnostics of star formation, metallicity, and ionization conditions in galaxies. Interpreting spatially resolved observations from integral field surveys (e.g., MaNGA , , ) and comparing them with hydrodynamical simulations requires three-dimensional photoionization models that handle realistic geometries, dust attenuation, and synthetic instrument output. VLT/MUSE JWST/NIRSpec We present a new photoionization module for the Monte Carlo radiative transfer code that predicts emission-line luminosities of ionized gas in three dimensions. It combines precomputed tables for gas temperature and opacity with a direct calculation of ion fractions and line emissivities. SKIRT Cloudy The module characterizes the local ionizing radiation field by log U and four spectral-shape ratios across the ionizing continuum (1--6,Ryd). Precomputed Cloudy tables map these to gas temperature and photoionization opacities, and they converge through 's existing iteration cycle. Emission-line luminosities are computed directly. An inline solver determines ion fractions from the converged radiation field and temperature and evaluates emissivities from recombination coefficients and collisional excitation rates. We validated the module against on 60 spherical shell models and against on a Milky Way-analog galaxy. SKIRT Cloudy COLT Across the one-dimensional benchmark grid, hydrogen recombination lines agree with Cloudy to within a few percent (Hα median ratio 0.97), and the forbidden lines to within ∼5%, except (median ratio 1.23), whose offset traces a temperature overestimate near the ionization front. In the three-dimensional galaxy comparison, integrated luminosities agree with COLT to within 18% for the hydrogen lines and 2% for while and are systematically elevated by ∼70% and ∼80%. Pixel-by-pixel correlation coefficients reach r ≥ 0.92, with luminosity-weighted scatter of 0.14--0.31,dex, and broadly consistent BPT line ratios. The module enables self-consistent synthetic observations in which ionized-gas emission lines, dust attenuation, and dust reemission are computed in a single Monte Carlo radiative transfer simulation applicable to any hydrodynamical simulation.