1999ApJ...519...55Y

CCD Hα images were used to study the properties of H II regions in 29 normal Im galaxies and six blue compact dwarf/starburst irregulars. The Hα emission line fluxes were measured and used to construct luminosity functions for each galaxy. For most galaxies, the luminosity function is well represented by a power law. Only two of the 29 Im galaxies have supergiant H II regions, which is probably a result of the small size and small total numbers of H II regions in irregular galaxies. BCDs and starburst galaxies, on the other hand, do not always have supergiant H II regions but may have them even if only a few regions are present. Comparison of the cumulative composite H II luminosity function with that of spiral galaxies shows that the irregulars in our sample do not have an unusually large population of supergiant H II regions relative to the galactic luminosity. Thus, these galaxies will not suffer a disproportionate amount of disruption by the concentration of massive stars within such regions. However, in BCD/starburst galaxies there is an excess of H II regions with luminosities greater than 10³⁸ ergs.s^–1 compared to Im and Sc galaxies relative to the luminosities of the host galaxies. Most of the H II region luminosity in normal Im galaxies comes from small regions and that in BCD/starburst galaxies from moderately large regions. Over 80% of the H II region luminosity in irregulars is found in complexes of H II regions typically with summed luminosities equivalent to ≤10 Orion nebulae. Two types of luminosity functions were observed: those that exhibit turnover and those that do not. Those that do exhibit turnover have an average slope of -1.5±0.1, and those that do not exhibit turnover tend to be complete to fainter levels, have lower upper luminosity cutoffs, have fewer H II regions, and have shallower slopes of -1.0±0.1. If the luminosity function is universal for a galaxy type, this difference suggests the possibility of a break in the luminosity function corresponding to H II regions ionized by single stars and H II regions ionized by clusters of stars. However, we cannot exclude the possibility that the two groups of galaxies are different in terms of their global star formation properties. Diameters of H II regions were also measured and diameter distributions were constructed for each of the galaxies. The diameter distribution was fitted with an exponential and the characteristic diameter D₀ was measured for each galaxy, although this fit did not characterize the distribution very well in most galaxies. The weak correlation between D₀ and the absolute blue magnitude of the galaxy suggested by others is confirmed to be very weak.