Hα Velocity Fields of H II Regions in Nearby Dwarf Irregular Galaxies

We present Hα velocity fields of 13 giant H II regions in four nearby dwarf irregular galaxies: NGC 2366, Holmberg II, IC 2574, and WLM (the Wolf-Lundmark-Melotte system). We classify the velocity features, as well as the morphologies, of the H II regions. The Hα velocity features are divided into three categories; three H II regions show chaotic features with a typical scale of variation of a few hundred parsecs in size and a few times 10 km s-1 in velocity, one region exhibits an expanding-bubble feature, and the remaining nine have calm velocity fields. There is a correlation between the Hα velocity features and the morphology of the H II regions. We measured the bulk motion of the H II regions relative to the ambient H I velocity for the H II regions with calm velocity fields and found a typical velocity difference of about 5 km s-1. We discuss a model for the origin of star-forming regions based on the presence of the velocity difference between Hα and H I gas, as well as the H I characteristics.