Size distributions of H II regions in galaxies. I - Irregular galaxies

The size distributions for the H II regions of 18 irregular and related galaxies are determined from Hα plates. Almost without exception the integral diameter distribution fits an exponential law of the form previously deduced by van den Bergh (1981). The slopes are not correlated with observational or instrumental parameters, and are thus intrinsic to the galaxies. They show a strong correlation with galaxy luminosity in the sense that the less luminous galaxies have the steeper slopes. Six galaxies in the sample have significantly anomalous largest H II regions, a fact that suggests that giant H II regions are a separate class of objects that are formed by a different mechanism from that normally governing H II region sizes.