The Evolutionary Status of Isolated Dwarf Irregular Galaxies I. UBV and H alpha Imaging Observations

van Zee, L., 2000, AJ, 119, 2757.

Abstract

The results of UBV and H alpha 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 to 400 kpc, and thus are unlikely to have had significant interactions within the last several Gyr. The new observations confirm that dwarf Irregular galaxies are blue systems, with median values for the sample of 0.42 +/- 0.05 in (B-V) and -0.22 +/- 0.6 in (U-B). Analysis of the derived surface photometry indicates that most of these systems can be well fit by single exponential disks, and have only minor color gradients. Furthermore, the observed HII regions are distributed sparsely across the optical disk. HII region luminosity functions were constructed for those systems with a sufficient number of HII regions; the derived power-law indices are generally shallow, with a mean value of -1.6 +/- 0.3.