Nebular Continuum and Line Emission in Stellar Population Synthesis Models
Nell Byler, Julianne J. Dalcanton, Charlie Conroy, Benjamin D. Johnson
TL;DR
This work addresses the need to accurately model nebular emission in galaxy SEDs by integrating a self-consistent nebular model into the Flexible Stellar Population Synthesis code using the photoionization code Cloudy. The approach ties the ionizing spectrum and gas-phase metallicity to the stellar population, producing simultaneous predictions for nebular continuum and emission lines as a function of age and metallicity, with results embedded as pre-computed tables for efficient synthesis of arbitrary SFHs. Key contributions include improved agreement with observed H II region line ratios, better Ne III/O II behavior, and compatibility with He II emission from high-redshift galaxies when incorporating rotating stellar models; including post-AGB phases helps reproduce LIER-like line ratios. The model enhances the reliability of SFR, metallicity, and stellar mass inferences from galaxy spectra and broadbands by providing a physically consistent link between stars, gas, and dust in SPS analyses.
Abstract
Accounting for nebular emission when modeling galaxy spectral energy distributions (SEDs) is important, as both line and continuum emission can contribute significantly to the total observed flux. In this work, we present a new nebular emission model integrated within the Flexible Stellar Population Synthesis code that computes the total line and continuum emission for complex stellar populations using the photoionization code Cloudy. The self-consistent coupling of the nebular emission to the matched ionizing spectrum produces emission line intensities that correctly scale with the stellar population as a function of age and metallicity. This more complete model of galaxy SEDs will improve estimates of global gas properties derived with diagnostic diagrams, star formation rates based on H$α$, and stellar masses derived from NIR broadband photometry. Our models agree well with results from other photoionization models and are able to reproduce observed emission from H II regions and star-forming galaxies. Our models show improved agreement with the observed H II regions in the Ne III/O II plane and show satisfactory agreement with He II emission from $z=2$ galaxies when including rotating stellar models. Models including post-asymptotic giant branch stars are able to reproduce line ratios consistent with low-ionization emission regions (LIERs).