D. Kunze , D. Rigopoulou , R. Genzel , & D. Lutz
Max-Planck Institut für Extraterrestrische Physik, Postfach 1603, D-85748 Garching, Germany
Molecular Hydrogen is one of the major constituents of the interstellar medium.
With the advent of the Infrared Space Observatory (ISO) it was for the first
time possible to observe the pure rotational lines of H2 in external galaxies.
As part of MPE's ISO guaranteed time program we have used the Short Wavelength
Spectrometer (SWS) to study these transitions between the low-lying energy
levels of H2 in a number of galaxies.
We present results of our survey of molecular Hydrogen line emission from 20 different galaxies, with types ranging from starburst and ultraluminous IRAS galaxies to AGNs. For each of the targets several H2 lines have been observed providing line fluxes or upper limits for typically the S(0), S(1), S(2) and S(5) transition.
Excitation diagrams are used to derive excitation temperatures for the warm molecular Hydrogen gas. The origin of the warm H2 gas, whether it is produced in shocks or UV fluoresence, is discussed. Additionally the properties of the H2 emission in starbursts and Seyferts and for the first time in molecular-gas rich ultraluminous IRAS galaxies are compared.