Turbulence in simulated HII regions

TL;DR

This study examines the complex turbulence within simulated HII regions, revealing that energy injection occurs across multiple scales, and evaluates observational methods for analyzing velocity fluctuations, highlighting the effectiveness of Velocity Channel Analysis.

Contribution

It demonstrates the scale-dependent turbulence in HII regions and assesses observational techniques, showing VCA's superiority over structure functions for velocity spectrum analysis.

Findings

Multiple energy injection scales flatten the fluctuation spectrum.

Structure function approach is unreliable for velocity spectrum.

VCA can accurately recover the velocity power spectrum slope.

Abstract

We investigate the scale dependence of fluctuations inside a realistic model of an evolving turbulent HII region and to what extent these may be studied observationally. We find that the multiple scales of energy injection from champagne flows and the photoionization of clumps and filaments leads to a flatter spectrum of fluctuations than would be expected from top-down turbulence driven at the largest scales. The traditional structure function approach to the observational study of velocity fluctuations is shown to be incapable of reliably determining the velocity power spectrum of our simulation. We find that a more promising approach is the Velocity Channel Analysis technique of Lazarian & Pogosyan (2000), which, despite being intrinsically limited by thermal broadening, can successfully recover the logarithmic slope of the velocity power spectrum to a precision of +-0.1 from high resolution optical emission line spectroscopy.