Star formation thresholds in low surface brightness galaxies

Low Surface Brightness (LSB) galaxies appear to have low star formation rates despite their often quite normal H I contents as judged from global H I properties such as M_{H I}/L and M_{H I}/M_T ratios. H I imaging with the Very Large Array of the National Radio Astronomy Observatory (the NRAO is operated by Associated Universities Inc. under contract with the National Science Foundation) of eight LSB galaxies shows that the H I is extended compared with the optical size and has average surface densities which are about a factor 2 lower than in High Surface Brightness (HSB) galaxies of the same type. The resolution of the H I imaging allows a rough rotation curve analysis for evaluating the critical density for star formation as formulated by Kennicutt [ApJ, 344, 685 (1989)]. The observed H I surface densities systematically fall below this critical density for most of the galaxies in this sample, in agreement with the low current star formation rates. From the optical surface photometry we conclude that the galaxies studied are in general late-type galaxies dominated by an exponential disk with a typical scale length of a few kpc. The B - R and V - I colors of the LSB galaxies are a few tenths of a magnitude bluer than those of HSB galaxies indicating that the disks of these galaxies have a mean young age.