The evolutionary status of isolated dwarf irregular galaxies. II. Star formation histories and gas depletion

The results of UBV and H alpha imaging of a large sample of isolated dwarf irregular galaxies are interpreted in the context of composite stellar popu lation models. The observed optical colors are best fitted by composite ste llar populations that have had approximately constant star formation rates for at least 10 Gyr. The galaxies span a range of central surface brightnes s, from 20.5 to 25.0 mag arcsec(-2); there is no correlation between surfac e brightness and star formation history. Although the current star formatio n rates are low, it is possible to reproduce the observed luminosities with out a major starburst episode. The derived gas depletion timescales are lon g, typically similar to 20 Gyr. These results indicate that dwarf irregular galaxies (dI's) will be able to continue with their slow, but constant, st ar formation activity for at least another Hubble time. The sample of isola ted dI's is compared with a sample of star-bursting dwarf galaxies taken fr om the literature. The star-bursting dwarf galaxies have many similar prope rties; the main difference between these two types of gas-rich dwarf galaxi es 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 g alaxies, 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 gradie nts indicates that star formation percolates slowly across the disks of nor mal dwarf galaxies in a quasi-continuous manner.