The dynamics and chemical composition of giant extragalactic H II regions

We analyse the correlations between Hβ luminosities, linear diameters and the widths of the global emission-line profiles of giant extragalactic H II regions and detached extragalactic H II regions. After correction for luminosity evolution, giant H II regions satisfy the same luminosity–velocity dispersion relation followed by elliptical galaxies, bulges of spiral galaxies and globular clusters. We also show that the correlation between size and line width is similar to the one valid for elliptical galaxies and globular clusters. These results strongly suggest that giant H II regions are self-gravitating systems, where the emission-line profile widths reflect the motions in the local gravitational field. The scatter observed in the Hβ luminosity–linewidth relation is found to be largely due to a metallicity effect. This implies that giant H II regions and detached extragalactic H II regions are at least a two-parameter family. After correction for this effect, the scatter in the corrected relation is less than 0.5 mag, consistent with the observational errors. The importance of these results as a new method for distance calibration and for the study of star formation and evolution in extragalactic systems is emphasized.