Comparison of Theoretical Starburst Photoionization Models for Optical Diagnostics

Abstract

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.