The internal dynamical equilibrium of H II regions: a statistical study

We present an analysis of the integrated Hα emission line profiles for the H  region population of the spiral galaxies NGC 1530, NGC 6951 and NGC 3359. We show that ∼70% of the line profiles show two or three Gaussian components. The relations between the luminosity (log LHα) and non–thermal line width (log σnt) for the H  regions of the three galaxies are studied and compared with the relation found taken all the H  regions of the three galaxies as a single distribution. In all of these distributions we find a lower envelope in log σnt. A clearer envelope in σnt is found when only those H  regions with σnt > σs(13 km s ) are considered, where σs is a canonical estimate of the sound speed in the interestellar medium. The linear fit for the envelope is log LHα = (36.8 ± 0.7) + (2.0 ± 0.5) log σnt where the Hα luminosity of the region is taken directly from a photometric H  region catalogue. When the Hα luminosity used instead is that fraction of the H  region luminosity, corresponding to the principal velocity component, i.e. to the turbulent non–expanding contribution, the linear fit is log LHα = (36.8 ± 0.6) + (2.0 ± 0.5) log σnt, i.e. unchanged but slightly tighter. The masses of the H  regions on the envelope using the virial theorem and the mass estimates from the Hα luminosity are comparable, which offers evidence that the H  regions on the envelope are virialized systems, while the remaining regions, the majority, are not in virial equilibrium.