# Comparison of Theoretical Starburst Photoionisation Models for Optical Diagnostics

## TL;DR

This study compares various stellar and nebular parameters in photoionisation models to understand their effects on optical emission lines and galaxy diagnostics, highlighting the importance of H II region properties.

## Contribution

It systematically evaluates how different stellar synthesis models and nebular parameters influence emission-line diagnostics and galaxy interpretations.

## Key findings

- Differences in emission-line ratios due to stellar model variations are small (~0.1 dex).
- Large differences in line ratios arise from intrinsic H II region parameters.
- Low-metallicity galaxies are better modeled as density-bounded H II regions.

## Abstract

We study and compare different examples of stellar evolutionary synthesis input parameters used to produce photoionisation model grids using the MAPPINGS V modelling 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) characterise 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 photoionisation models. We explore and compare the stellar input ionising 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, ionisation parameter, pressure, H II region boundedness). We also perform a comparison of the photoionisation codes MAPPINGS and CLOUDY. On the variations in the ionising 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.