Molecular gas in spiral galaxies

Molecular clouds are the active star forming component of the interstellar medium, and as such are critically important to the evolution of galaxies. The theoretical and empirical basis for using the 2.6 mm CO emission line as a tracer of H2 is presented, followed by a summary of the properties and distribution of molecular clouds in the Galaxy. The radial distributions of H2 in nearby spiral galaxies are compared with that of the Milky Way and recent observations of molecular clouds vis a vis the arms of spiral galaxies are then discussed. Lastly, we review the properties of molecular gas in ultraluminous infrared galaxies which are currently thought to be undergoing bursts of star formation and which may be evolving to become active galactic nuclei. (author)