Supersonic Turbulence in Giant Extragalactic HII Regions

Summary

The physical mechanism responsible for the supersonic broadening of the integrated emission lines of Giant HII Regions (GHR) to velocities well above the sound speed of the ionized gas is yet not clear. The observational evidence is reviewed and possible physical mechanisms discussed in this paper. It is shown that hydrodynamical turbulence and thermal motions dominate the kinematics of the gas at small scales while gravity and stellar winds are responsible for the width of the integrated line-profiles. The relative contribution of these two dominant mechanisms depends on age. Gravity dominates in young nebulae whereas expanding shells dominate when the most massive stars become supergiants.

Introduction

More than their large sizes, the key defining property of Giant HII regions (GHIIRs), as a distinct class of objects, is the supersonic velocity widths of their integrated emissionline profiles (Smith & Weedman 1972; Melnick 1977; Melnick et al. 1987 and references therein). Since supersonic gas motions will rapidly decay due to the formation of strong radiative shocks, the detection of Mach numbers greater than 1 in the nebular gas poses an astrophysically challenging problem.

Melnick (1977) suggested that the ionized gas is made of dense clumps moving in an empty or very tenuous medium, so that the integrated profiles reflect the velocity dispersion of discrete clouds rather than hydrodynamical turbulence. In this model, the relevant time scale for radiative decay of the kinetic energy is the crossing-time of the HII regions which turns out to be comparable to the ages of the ionizing clusters.