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

Reliable nebular emission line diagnostics are essential for accurately inferring the physical properties (e.g. electron temperature, density, pressure, and metallicity) of H~{\small 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~{\small 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~{\small 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~{\small II} regions exhibit systematically higher [N~{\small II}]/H\(\alpha\), lower [O~{\small III}]/H\(\beta\), and lower O32, compared to homogeneous H~{\small 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~{\small II} regions models that span the range of conditions seen at both high and low redshift to better interpret observed spectra.