Hα line profiles for a sample of supergiant HII regions I.: The main spectral component

We present an analysis of the Ha emission line profiles of a sample of giant extragalactic HII regions, selected from among the brightest and most isolated in a group of spiral galaxies for which we have photometric and spectroscopic data: NGC 157, NGC 3631, NGC 6764, NGC 3344, NGC 4321, NGC 5364, NGC 5055, NGC 5985, NGC 7479. Our study confirms that the majority of the line profiles are composed of a bright, main component and two fainter, high velocity components that we denominate wings. Here, we analyze the kinematics of the principal components, finding a relation between the Ha luminosity, L-Ha, and the turbulent velocity dispersion, sigma(nt). A linear fit to the relation between these quantities yields log L-H alpha = (35.6 +/- 0.4) + (2.87 +/- 0.2) log sigma (nt), in agreement with previous studies. We compute the mass of each HII region using both the virial theorem and the Ha luminosity, confirming that, though these estimates do not coincide exactly, they are comparable within the uncertainties and consequently that the HII regions in our sample are approximately virialized.