Gas Mass Fractions and the Evolution of Low Surface Brightness Dwarf Galaxies

The optical and H I properties are presented for a sample of low surface brightness (LSB) dwarf galaxies, cataloged from the Second Palomar Sky Survey. Gas mass fractions for LSB dwarfs reach the highest levels of any know galaxy type (fg = 95%), confirming that their low stellar densities are due to inefficient conversion of gas mass into stellar mass. Comparison with star formation models indicates that the blue optical colors of LSB dwarfs are not due to low metallicity or to recent star formation and can only be explained by a dominant stellar population less than 5 Gyr in mean age. If star formation occurs in OB complexes, as in normal galaxies, then LSB dwarfs must undergo weak bursts traveling over the extent of the galaxy to maintain their LSB nature, which contributes to their irregular morphological appearance.