Inferring Interstellar Medium Density, Temperature, and Metallicity from Turbulent H II Regions

TL;DR

This study uses 3D simulations to show how turbulence in H II regions affects nebular emission lines and the inferred physical properties, highlighting biases in metallicity and density diagnostics.

Contribution

The paper introduces detailed 3D turbulent H II region models to quantify how turbulence influences emission line diagnostics and property inferences.

Findings

Turbulence causes systematic shifts in emission line ratios.

Metallicity estimates are biased low by up to 0.1 dex due to turbulence.

Different density indicators yield inconsistent electron densities.

Abstract

Reliable nebular emission line diagnostics are essential for accurately inferring the physical properties (e.g. electron temperature, density, pressure, and metallicity) of H II regions from spectra. When interpreting spectra, it is typical to adopt a single zone model, e.g. at fixed density, pressure, or temperature, to infer H II region properties. However, such an assumption may not fully capture the complexities of a turbulent interstellar medium. To understand how a complex density field driven by supersonic turbulence impacts nebular emission lines, we simulate 3D H II regions surrounding a single O star, both with and without supersonic turbulence. We find that turbulence directly impacts the values of common strong line ratios. For example turbulent H II regions exhibit systematically higher [N II]/H$\alpha$, lower [O III]/H$\beta$, and lower O32, compared to homogeneous H II regions with the same mean density and ionizing source. These biases can impact inferences of metallicity, ionization parameter, excitation, and ionization source. For our choice of turbulence, direct $T_e$ method metallicity inferences are biased low, by up to 0.1 dex, which is important for metallicity studies, but not enough to explain the abundance discrepancy problem. Finally, we show that large differences between measured electron densities emerge between infrared, optical, and UV density indicators. Our results motivate the need for large grids of turbulent H II regions models that span the range of conditions seen at both high and low redshift to better interpret observed spectra.