The internal dynamical equilibrium of HII regions:: A statistical study

We present an analysis of the integrated Halpha emission line profiles for the H II region population of the spiral galaxies NGC 1530, NGC 6951 and NGC 3359. We show that similar to70% of the line profiles show two or three Gaussian components. The relations between the luminosity (log L-Halpha) and non-thermal line width (log sigma(nt)) for the H II regions of the three galaxies are studied and compared with the relation found taken all the H II regions of the three galaxies as a single distribution. In all of these distributions we find a lower envelope in log sigma(nt). A clearer envelope in sigma(nt) is found when only those H II regions with sigma(nt) > sigma(s)(13 km s(-1)) are considered, where sigma(s) is a canonical estimate of the sound speed in the interestellar medium. The linear fit for the envelope is log L-Halpha = (36.8 +/- 0.7) + (2.0 +/- 0.5) log sigma(nt) where the Halpha luminosity of the region is taken directly from a photometric H II region catalogue. When the Halpha luminosity used instead is that fraction of the H II region luminosity, corresponding to the principal velocity component, i.e. to the turbulent non-expanding contribution, the linear fit is log L-Halpha = (36.8 +/- 0.6) + (2.0 +/- 0.5) log sigma(nt), i.e. unchanged but slightly tighter. The masses of the H II regions on the envelope using the virial theorem and the mass estimates from the Halpha luminosity are comparable, which offers evidence that the H II regions on the envelope are virialized systems, while the remaining regions, the majority, are not in virial equilibrium.