Supersonic Motions in Giant HII Regions

Summary

There are several questions in the field of the study of giant extragalactic HII regions (GEHRs) that remain controversial. The origin of the observed supersonic motions and the validity of the relationship between size and velocity dispersion obtained on the base of single-aperture observations are still debated. From a purely observational point of view, the spatial extent of kinematical features within the nebula, the sense in assuming Gaussian profiles as representative of the variety of emision lines found in GEHR, and finally how lawful the assignment of a single value (σ, R) as a defining kinematical parameter of a GEHR are also questioned. The energy input required to provide the supersonic motions observed in GEHRs has been the subject of several scenarios proposed in the literature and some of the points in favour and/or against them are mentioned in this contribution.

Bidimensional spectroscopy with good spatial and spectral resolution, sampling of a particular emision line over the whole emitting area, is shown to be the most suitable observational technique for understanding the global kinematics of GEHRs. In NGC 604 for instance, features like loops and filaments although clearly seen in deep exposures and obviously resulting from stellar winds and SN explosions, do not dominate the bulk of the emission that comes from smaller but much brighter areas. Thus the total flux from split or non-Gaussian profiles that arise from shells and/or filaments is much lower than that obtained from bright knots where line profiles are well fitted by Gaussians.