Comparison of Theoretical Starburst Photoionization Models for Optical Diagnostics

We study and compare different examples of stellar evolutionary synthesis input parameters used to produce photoionization model grids using the MAPPINGS V modeling code. The aim of this study is to (a) explore the systematic effects of various stellar evolutionary synthesis model parameters on the interpretation of emission lines in optical strong-line diagnostic diagrams, (b) characterize the combination of parameters able to reproduce the spread of local galaxies located in the star-forming region in the Sloan Digital Sky Survey, and (c) investigate the emission from extremely metal-poor galaxies using photoionization models. We explore and compare the stellar input ionizing spectrum (stellar population synthesis code [Starburst99, SLUG, BPASS], stellar evolutionary tracks, stellar atmospheres, star formation history, sampling of the initial mass function), as well as parameters intrinsic to the H ii region (metallicity, ionization parameter, pressure, H ii region boundedness). We also perform a comparison of the photoionization codes MAPPINGS and CLOUDY. On the variations in the ionizing spectrum model parameters, we find that the differences in strong emission-line ratios between varying models for a given input model parameter are small, on average ∼0.1 dex. An average difference of ∼0.1 dex in emission-line ratio is also found between models produced with MAPPINGS and CLOUDY. Large differences between the emission-line ratios are found when comparing intrinsic H ii region parameters. We find that low-metallicity galaxies are better explained by a density-bounded H ii region and higher pressures better encompass the spread of galaxies at high redshift.