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 H2 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 H2 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 H2 excitation in extragalactic environments. Finally, we find that although PAH and H2 line emission correlate well in Starbursts and the large scale emission in AGN, H2 emission is much stronger compared to PAH emission in cases where a “pure” AGN dominates the energy output.