The Evolutionary Status of Isolated Dwarf Irregular Galaxies. I. [ITAL]UBV[/ITAL] and Hα Imaging Observations

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Abstract
The results of UBV and Ha 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 200È400 kpc and thus are unlikely to have had signiÐcant interactions within the last several gigayears. The new observations conÐrm that dwarf irregular galaxies are blue systems, with median values for the sample of in B[V and in U[B. Analysis of the derived surface 0.42^0 .05 0.04 [0.22^0 .07 0.04 photometry indicates that most of these systems can be well Ðtted by single exponential 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 number of H II regions ; the derived power-law indices are generally shallow, with a median value of [1.61^0 .24 0.31.

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