The molecular ISM in low surface brightness disk galaxies

We present models for the interstellar medium in disk galaxies. In particul ar, we investigate whether the ISM in low surface brightness galaxies can s upport a significant fraction of molecular gas given their low metallicity and surface density. It is found that the abundance and line brightness of CO in LSB galaxies is small and typically below current observational limit s. Still, depending on physical details of the ISM, the fraction of gas in the form of molecular hydrogen can be significant in the inner few kilopars ecs of a low surface brightness galaxy. This molecular gas would be at temp eratures of similar to 3-50 K, rather higher than in high surface brightnes s galaxies. These results may help explain the star-forming properties and inferred evolutionary history of LSB galaxies.