The Stellar Content of 10 Dwarf Irregular Galaxies

Abstract

We discuss the morphologies, color-magnitude diagrams, and frequencies of the resolved stars together with the morphology of the ionized gas, as well as the surface brightness profiles and colors of the underlying light distributions of unresolved stars. We point out the occurrence in half of the galaxies studied of H II regions and young stellar associations located well outside of the main body of resolved stars. This appears to be in conflict with the hypothesis of self-propagating star formation. All of the sample galaxies contain H II regions and young massive stars with ages of a few Myr to around 10 Myr. For supergiants beyond an age of about 50 Myr, incompleteness is already a problem in the single-star photometry. However, we can also gain insight into the stellar content from the integrated colors of the unresolved stars. The light distribution of the unresolved stars is more extended than that of the resolved stars and is of a more regular and elliptical shape. We provide ellipticities, central surface brightnesses and scale lengths for the sample galaxies. The background-light colors indicate a range of star formation histories for the sample galaxies, with galaxy colors at one extreme being dominated by the old, metal-poor population and at the other extreme by the most recent star-birth event. The results provide insight into the stellar content and the star formation histories of isolated, late-type galaxies.