Physics
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...421..115d&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 421, no. 1, p. 115-121
Physics
20
Emission Spectra, Fluorescence, Galaxies, Hydrogen Ions, Infrared Spectroscopy, Near Infrared Radiation, Photons, Stellar Radiation, Interstellar Matter, Molecular Clouds, Spectrum Analysis, Star Formation, Stellar Physics
Scientific paper
The excitation mechanism and origin of the molecular hydrogen (H2) observed in the starburst galaxy NGC 3256 is discussed. The relative intensities of K-window H2 transitions suggest that roughly half of the 1 goes to 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 goes to 0 S(1) flux. This implies that UV fluorescence is responsible for more than 90% of the total H2 emission emitted at all wavelengths. The potential contribution of shock-excited H2 emission is also investigated. A starburst model is used to predict the time evolution of the 1 goes to 0 S(1)/Br-gamma ratio expected from an ensemble of star-forming regions in which the H2 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 approximately 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 goes to 0 S(1)/Br-gamma ratio, we conclude that less than 30% of the total 1 goes to 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 H2 emission in comparable quantities is a natural consequence of starburst activity.
Doyon Rene
Joseph Robert D.
Wright Gillian S.
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