Properties of H II Region Populations in Galaxies. I. The First-ranked H II Regions

Hα photometry of the brightest H II regions in 95 nearby spiral and irregular galaxies has been used to investigate the dependence of the H II region populations on the properties of their parent galaxies. The first-ranked H II regions span a range of four orders of magnitude in luminosity, and their properties are correlated with luminosities and Hubble types of their parent galaxies. The brightest H II regions in Magellanic irregular galaxies are approximately 50 times brighter than those in early-type spirals of the same absolute magnitude. This change results from both an increase in the numbers of H II regions in late-type galaxies and a physical change in the characteristic sizes of the star-forming regions. Similar trends are observed in the diameters of the brightest H II regions and in the magnitudes of the first-ranked stellar associations. The properties of the first-ranked H II regions are also correlated with absolute magnitude, but these correlations can be explained as arising from simple size-of-sample selection effects.