SIMBAD references

We examine the stellar content of 10 dwarf irregular galaxies, the broadband CCD photometry of which was published in 1995 by Hopp & Schulte-Ladbeck. We also present Hα images for several of these galaxies. The galaxies in the sample are located outside of the Local Group. Yet they are still close enough to be resolved into single stars from the ground but only the brightest stars (or star clusters) are detected and there is severe crowding. The sample galaxies were selected to be isolated from massive neighbors; about half of them are (mostly peripheral) members of groups, the other half is located in the field. We discuss the vicinity of the sample galaxies to other dwarf galaxies.

In order to interpret single-star photometry and draw conclusions about the stellar content or other distance-dependent quantities, it is crucial that accurate distances to the galaxies be known. The distances to the sample galaxies are not well known since all but one have not had a primary distance indicator measured. We make an attempt to constrain the distances by identifying the envelope of the brightest supergiants in B, B-R and R, B-R color-magnitude diagrams, but the results are not very accurate (we estimate the minimal error on the distance modulus is 1.36 mag). Nevertheless, the fact that the sample galaxies are resolved with direct ground-based imaging indicates that they are sufficiently nearby to represent good candidates for observations with instruments that provide high spatial resolution, e.g., adaptive optics systems on large ground-based telescopes, or the Hubble Space Telescope.

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.