Fluorescent molecular hydrogen emission from star-forming galaxies

Near-infrared molecular hydrogen emission has been detected from the central star-forming complexes in nearby spiral galaxies. In all cases, measurement of several transitions demonstrates than the bulk of the line-emitting gas is excited by UV radiation, and not by shocks. For most galaxies, the ratio of the molecular (v=1-0 S(1)) to atomic (Brγ) hydrogen line strengths can be quantitatively understood if the emitting regions consist of individual hot stars (or small clusters) embedded within the parent molecular clouds or forming blisters at their surface. Large central star clusters may account for the handful of galaxies which display relatively weak H2 emission