The Evolutionary Status of Isolated Dwarf Irregular Galaxies. II. Star Formation Histories and Gas Depletion

The results of UBV and Hα imaging of a large sample of isolated dwarf irregular galaxies are interpreted in the context of composite stellar population models. The observed optical colors are best fitted by composite stellar populations that have had approximately constant star formation rates for at least 10 Gyr. The galaxies span a range of central surface brightness, from 20.5 to 25.0 mag arcsec -2 ; there is no correlation between surface brightness and star formation history. Although the current star formation rates are low, it is possible to reproduce the observed luminosities without a major starburst episode. The derived gas depletion timescales are long, typically ∼20 Gyr. These results indicate that dwarf irregular galaxies (dI's) will be able to continue with their slow, but constant, star formation activity for at least another Hubble time. The sample of isolated dI's is compared with a sample of starbursting dwarf galaxies taken from the literature. The starbursting dwarf galaxies have many similar properties; the main difference between these two types of gas-rich dwarf galaxies is that the current star formation is concentrated in the center of the starbursting systems, while it is much more distributed in the quiescent dI's. This results in pronounced color gradients for the starbursting dwarf galaxies, while the majority of the quiescent dwarf irregular galaxies have minor or nonexistent color gradients. Thus, the combination of low current star formation rates, blue colors, and the lack of significant color gradients indicates that star formation percolates slowly across the disks of normal dwarf galaxies in a quasi-continuous manner.