An ISO--SWS survey of molecular hydrogen in Starburst and Seyfert Galaxies

We present results from a survey of molecular hydrogen emission from a sample of Starburst and Seyfert galaxies carried out with the Infrared Space Observatory (ISO). Pure rotational H_2 emission has been detected in a number of extragalactic objects and a variety of environments. A number of transitions from S(7) to S(0) are detected in both Starbursts and Seyferts. Using excitation diagrams we derive temperatures and masses of the ``warm'' molecular hydrogen. We find that the temperature of the ``warm'' gas is similar in Starbursts and Seyferts (those Seyferts for which we have firm detections of the S(0) line) with a value of around T~150 K. This ``warm'' gas accounts for as much as 10% of the total galactic mass (as probed by CO molecular observations) in Starbursts. The fraction of ``warm'' gas is overall higher in Seyferts, ranging between 2--35%. We then investigate the origin of the warm H_2 emission. Comparison with published theoretical models and Galactic templates implies that although emission from photodissociation regions (PDR) alone could explain the emission from Starbursts and Seyferts, most likely a combination of PDR, shock emission and gas heated by X-rays (mostly for the Seyferts) is responsible for H_2 excitation in extragalactic environments. Finally, we find that although PAH and H_2 line emission correlate well in Starbursts and the large scale emission in AGN, H_2 emission is much stronger compared to PAH emission in cases where a ``pure'' AGN dominates the energy output.