Comparison of Theoretical Starburst Photoionization Models for Optical Diagnostics

Abstract

We study and compare different examples of stellar evolutionary synthesis\ninput parameters used to produce photoionisation model grids using the MAPPINGS\nV modelling code. The aim of this study is to (a) explore the systematic\neffects of various stellar evolutionary synthesis model parameters on the\ninterpretation of emission lines in optical strong-line diagnostic diagrams,\n(b) characterise the combination of parameters able to reproduce the spread of\nlocal galaxies located in the star-forming region in the Sloan Digital Sky\nSurvey, and (c) investigate the emission from extremely metal-poor galaxies\nusing photoionisation models. We explore and compare the stellar input ionising\nspectrum (stellar population synthesis code [Starburst99, SLUG, BPASS], stellar\nevolutionary tracks, stellar atmospheres, star-formation history, sampling of\nthe initial mass function) as well as parameters intrinsic to the H II region\n(metallicity, ionisation parameter, pressure, H II region boundedness). We also\nperform a comparison of the photoionisation codes MAPPINGS and CLOUDY. On the\nvariations in the ionising spectrum model parameters, we find that the\ndifferences in strong emission-line ratios between varying models for a given\ninput model parameter are small, on average ~0.1 dex. An average difference of\n~0.1 dex in emission-line ratio is also found between models produced with\nMAPPINGS and CLOUDY. Large differences between the emission-line ratios are\nfound when comparing intrinsic H II region parameters. We find that\nlow-metallicity galaxies are better explained by a density-bounded H II region\nand higher pressures better encompass the spread of galaxies at high redshift.\n