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We present results from a survey of molecular hydrogen emission from a sampleof Starburst and Seyfert galaxies carried out with the Infrared SpaceObservatory (ISO). Pure rotational H_2 emission has been detected in a numberof extragalactic objects and a variety of environments. A number of transitionsfrom S(7) to S(0) are detected in both Starbursts and Seyferts. Usingexcitation diagrams we derive temperatures and masses of the ``warm'' molecularhydrogen. We find that the temperature of the ``warm'' gas is similar inStarbursts and Seyferts (those Seyferts for which we have firm detections ofthe S(0) line) with a value of around T~150 K. This ``warm'' gas accounts foras much as 10% of the total galactic mass (as probed by CO molecularobservations) in Starbursts. The fraction of ``warm'' gas is overall higher inSeyferts, ranging between 2--35%. We then investigate the origin of the warmH_2 emission. Comparison with published theoretical models and Galactictemplates implies that although emission from photodissociation regions (PDR)alone could explain the emission from Starbursts and Seyferts, most likely acombination of PDR, shock emission and gas heated by X-rays (mostly for theSeyferts) is responsible for H_2 excitation in extragalactic environments.Finally, we find that although PAH and H_2 line emission correlate well inStarbursts and the large scale emission in AGN, H_2 emission is much strongercompared to PAH emission in cases where a ``pure'' AGN dominates the energyoutput.Comment: Astronomy and Astrophysics in pres

An ISO–SWS survey of molecular hydrogen in starburst and Seyfert galaxies