The evolutionary status of isolated dwarf irregular galaxies. I. UBV and Halpha imaging observations

The results of UBV and Her imaging of a large sample of gas-rich, isolated, dwarf irregular galaxies are presented. The majority of the low-luminosity systems in this sample have no known neighbors within 200-400 kpc and thus are unlikely to have had significant interactions within the last several gigayears. The new observations confirm that dwarf irregular galaxies are b lue systems, with median values for the sample of 0.42+/-(0.04)(0.05) in B- V and -0.22+/-(0.04)(0.07) in U-B. Analysis of the derived surface photomet ry indicates that most of these systems can be well fitted by single expone ntial disks and have only minor color gradients. Furthermore, the observed H II regions are distributed sparsely across the optical disk. H II region luminosity functions were constructed for those systems with a sufficient n umber of H II regions; the derived power-law indices are generally shallow, with a median value of -1.61+/-(0.31)(0.24).