Discovery of molecular hydrogen fluorescence in the diffuse interstellar medium

We report the first detection of molecular hydrogen fluorescence in the diffuse interstellar medium. Using the Berkeley UVX Shuttle Spectrometer, we have observed H-2 Lyman band fluorescence in four directions, each with high significance. This is the first direct observation of the radiative byproduct of H-2 photodissociation in the diffuse interstellar medium. Molecular hydrogen fluorescence is detected in all directions that have previously been found to contain significant CO emission. We have developed a simple equilibrium model that includes attenuation of the incident UV radiation field by H-2 line and dust continuum absorption. We find that these effects must be included to obtain an accurate prediction of the emergent flux. We use this to compare the observed intensities to the observed CO and inferred H-2 column densities. We find evidence that the gas in the CO emission portions of the clouds may be clumpy, with a filling factor f < 0.2 and an average density [n(H)] > 30 cm(-3) in most cases. There is also evidence for an extended halo of molecular gas that is not detected in CO but still produces significant H-2 fluorescence. In the case of the Ursa Major high-latitude molecular clouds, we suggest that the clumpy core and diffuse halo indicate that the cloud is currently being destroyed by the radiation field.