A NEAR-INFRARED SPECTROSCOPIC STUDY OF THE LUMINOUS MERGER NGC-3256 .2. EVIDENCE FOR FLUORESCENT MOLECULAR-HYDROGEN EMISSION

The excitation mechanism and origin of the molecular hydrogen (H-2) ob served in the starburst galaxy NGC 3256 is discussed. The relative int ensities of K-window H-2 transitions suggest that roughly half of the 1-0 S(1) flux measured on the nucleus is fluorescently excited by UV p hotons. From a simple geometrical representation of the interstellar m edium, in which molecular clouds are bathed in a diffuse UV radiation field, we show that there are enough OB stars and molecular material i n the center of the galaxy to reproduce at least half if not all of th e observed 1-0 S(1) flux. This implies that UV fluorescence is respons ible for more than 90% of the total H-2 emission emitted at all wavele ngths. The potential contribution of shock-excited H-2 emission is als o investigated. A starburst model is used to predict the time evolutio n of the 1-0 S(1)/Br gamma ratio expected from an ensemble of star-for ming regions in which the H-2 flux is contributed only by young stella r (Orion-type) objects and supernova remnants. The model can reproduce line ratios in the range of 0.5-1.0, as typically observed in starbur st galaxies, provided that the starburst event is older than similar t o 30 million years and the star formation rate is exponentially decrea sing. The predictions of the model are valid only for a single-event s tarburst. Given the age of the burst inferred for NGC 3256 and the obs erved 1-0 S(1)Br gamma ratio, we conclude that less than 30% of the to tal 1-0 S(1) is contributed by young stellar objects and supernova rem nants in this galaxy. These results show that the production of both s hock- and fluorescently excited H-2 emission in comparable quantities is a natural consequence of starburst activity.