GALEX observations of low surface brightness galaxies:: UV color and star formation efficiency

We present GALEX UV observations of a sample of low surface brightness (LSB) galaxies for which H I data are available, allowing us to estimate their star formation efficiency. We find that the UV light extends to larger radii than the optical light ( some galaxies, but not all, look similar to the recently discovered XUV-disk galaxies). Using a standard calibration to convert the UV light into star formation rates, we obtain lower star formation efficiencies in LSB galaxies than in high surface brightness galaxies by about one order of magnitude. We show, however, that standard calibrations may not apply to these galaxies, as the FUV-NUV color obtained from the two GALEX bands (FUV and NUV; lambda(eff) = 1516 and 2267 angstrom, respectively) is redder than expected for star-forming galaxies. This color can be interpreted as a result of internal extinction, modified initial mass function, or star formation histories characterized by bursts followed by quiescent phases. Our analysis favors this latter hypothesis.