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 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$α$, lower [O III]/H$β$, 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.

💡 Research Summary

This paper investigates how supersonic turbulence, which creates a complex density field in the interstellar medium, influences the nebular emission‑line diagnostics commonly used to infer the physical conditions of H II regions. The authors perform three‑dimensional radiation‑hydrodynamics simulations with the RAMSES‑RTZ code coupled to the PRISM ISM model. Two sets of simulations are compared: (1) homogeneous boxes with constant density (ranging from 10⁻¹ to 10⁴ cm⁻³) and a single O4V star (T≈42,900 K, Q≈1.25×10⁵⁰ s⁻¹) at the centre, and (2) turbulent boxes where the same average density (300 cm⁻³) is perturbed by driven supersonic turbulence (Mach ≈ 5.5) with a natural mix of compressive and solenoidal modes. Ten independent turbulent density realizations are generated for each metallicity (0.05–1 Z⊙), frozen, and then illuminated by the same ionizing source until a steady radiative equilibrium is reached.

Emission lines are calculated on a cell‑by‑cell basis using electron temperature, density, and ion fractions from the simulation. Optical and UV lines are post‑processed with PyNeb (CHIANTI v10.1 atomic data), while mid‑ and far‑IR lines are computed directly in RAMSES‑RTZ using a cooling routine calibrated against CLOUDY. The authors examine classic density diagnostics (e.g.,