Morphology and star formation in nearby low surface brightness galaxies

We present observations (B, R, K, H alpha and H-I) of six nearby low surface brightness galaxies (LSBGs). They show an astonishing amount of variety; while some systems appear smooth and featureless, others resolve into loose assemblies of gas clouds. We have derived rotation curves, gas surface density profiles and star formation thresholds for three of the galaxies. The results have been used to test two ideas describing their star formation: one in which star formation depends solely on the H-I gas surface density, and one that depends on differential rotation. We find that a critical H-I surface density criterion in the range 2.6-12.6 x 10(20) cm(-2) (2.1-10.1 M-circle dot pc(-2)) best describes the star-forming ability of these galaxies on local and global scales. A critical gas surface density based on the rotation of the gas is also able to describe the results on a global scale for two of the three galaxies for which we were able to derive rotation curves.