Line Emission from Radiation-Pressurized HII Region II: Dynamics and Population Synthesis

TL;DR

This study models HII regions considering radiation pressure and winds, revealing their significant impact on emission line ratios and potential explanation for observed differences in galaxy spectra across cosmic time.

Contribution

It introduces a population synthesis model that incorporates radiation pressure effects, challenging previous wind-dominated assumptions in HII region modeling.

Findings

Line ratios can exceed previous theoretical limits.

Radiation pressure significantly influences galactic emission lines.

Possible explanation for high-redshift galaxy line ratio offsets.

Abstract

Optical and infrared emission lines from HII regions are an important diagnostic used to study galaxies, but interpretation of these lines requires significant modeling of both the internal structure and dynamical evolution of the emitting regions. Most of the models in common use today assume that HII region dynamics are dominated by the expansion of stellar wind bubbles, and have neglected the contribution of radiation pressure to the dynamics, and in some cases also to the internal structure. However, recent observations of nearby galaxies suggest that neither assumption is justified, motivating us to revisit the question of how HII region line emission depends on the physics of winds and radiation pressure. In a companion paper we construct models of single HII regions including and excluding radiation pressure and winds, and in this paper we describe a population synthesis code that uses these models to simulate galactic collections of HII regions with varying physical parameters. We show that the choice of physical parameters has significant effects on galactic emission line ratios, and that in some cases the line ratios can exceed previously claimed theoretical limits. Our results suggest that the recently-reported offset in line ratio values between high-redshift star-forming galaxies and those in the local universe may be partially explained by the presence of large numbers of radiation pressured-dominated HII regions within them.