1 Introduction

Using the Short Wavelength Spectrometer (SWS; De Graauw et al. 1996) on board the Infrared Space Observatory (ISO; Kessler et al. 1996), we have performed a survey of molecular hydrogen emission from active galaxies displaying a wide range in nuclear activity including pure bona-fide starbursts, Seyfert 2s (some of them with starburst components) and pure Seyfert 1 galaxies. Prior to the ISO mission, extragalactic H₂ emission had only been detected in the ro-vibrational lines around 2.1 $\mu$m. Indeed, H₂ ro-vibrational emission has been detected in galactic sources (e.g. Usuda et al. 1996), Starburst galaxies (e.g. Joseph et al. 1984), Seyferts (e.g. Moorwood & Oliva 1988; Fischer et al. 1987) and bright spirals (e.g. Puxley et al. 1988).

The ro-vibrational lines typically trace H₂ gas of masses around $\sim$10 $^{4-5}~ M_{\odot}$ and temperatures $\sim$2000 K. This gas can be excited either by collisions (thermal) or by absorption of ultraviolet (UV) photons in the Lyman and Werner electronic bands (912-1108 Å), followed by a de-excitation cascade to the ground state (fluorescence). However, gas at these temperatures is a very small fraction (as small as 10^-6) of the total amount of H₂ gas (e.g. Van der Werf et al. 1993). Since the ro-vibrational lines tend to get faint at lower temperatures, most of our knowledge about the H₂ content of galaxies comes from CO observations assuming a CO/H₂ conversion factor derived from galactic molecular cloud observations. ISO gave the unique opportunity to observe intermediate temperature gas, ie "warm'' H₂, directly in pure rotational lines. Since transitions with $\Delta J= \pm 1$ are strictly forbidden for the H₂molecule, the rotational ladder consists only of an ortho (J odd) and a para (J even) series of quadrupole transitions.

ISO, and in particular SWS, offered the unique opportunity to detect pure rotational H₂ emission in a number of galactic and extragalactic sources, thus studying the amount of moderately warm gas in these sources. The spectral range of SWS provides full coverage of a number of transitions (for most galaxies we have observed from the S(0) to S(7) transitions) while its spectral resolution is well matched to the typical velocity dispersions of galaxies. Among the first detections of pure rotational H₂ emission in galaxies were the detections in NGC 3256 (Rigopoulou et al. 1996), NGC 6946 (Valentijn et al. 1996), NGC 891 (Valentijn & Van der Werf 1999). However, no study of pure rotational H₂emission for a large number of galaxies has so far appeared.

Here, we present an inventory of H₂ emission lines from a number of Starburst and AGN. Our survey includes 12 and 9 Starburst and Seyfert galaxies, respectively. Temperatures of the warm molecular gas are deduced from excitation diagrams whereas the masses of the warm molecular gas are compared to the total gas content of the galaxies as estimated from molecular CO observations. The H₂ excitation mechanism is investigated next. The observations are compared to the predictions of published models both for PDR, shocked emission and X-ray irradiated gas, as well as to Galactic templates. Finally we examine possible correlations between PAH and H₂ emission.