The stellar content of 10 dwarf irregular galaxies

We examine the stellar content of 10 dwarf irregular galaxies of which broad-band CCD photometry was published in Hopp & Schulte-Ladbeck (1995). We also present Halpha 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. ...