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) observed in the starburst galaxy NGC 3256 is discussed. The relative intensities 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 photons. From a simple geometrical representation of the interstellar medium, in which molecular clouds are bathed in a diffuse UV radiation field, we show that there are enough OB stars and molecular material in the center of the galaxy to reproduce at least half if not all of the observed 1-0 S(1) flux. This implies that UV fluorescence is responsible for more than 90% of the total H-2 emission emitted at all wavelengths. The potential contribution of shock-excited H-2 emission is also investigated. A starburst model is used to predict the time evolution of the 1-0 S(1)/Br gamma ratio expected from an ensemble of star-forming regions in which the H-2 flux is contributed only by young stellar (Orion-type) objects and supernova remnants. The model can reproduce line ratios in the range of 0.5-1.0, as typically observed in starburst galaxies, provided that the starburst event is older than similar to 30 million years and the star formation rate is exponentially decreasing. The predictions of the model are valid only for a single-event starburst. Given the age of the burst inferred for NGC 3256 and the observed 1-0 S(1)Br gamma ratio, we conclude that less than 30% of the total 1-0 S(1) is contributed by young stellar objects and supernova remnants in this galaxy. These results show that the production of both shock- and fluorescently excited H-2 emission in comparable quantities is a natural consequence of starburst activity.