MOLECULAR-HYDROGEN EMISSION IN GALAXIES - DETERMINATION OF EXCITATIONMECHANISM

Infrared H-2 emission is a mixture of thermal and nonthermal component s. We have found the excitation temperature of the thermal component t o be 2000 K for supernova remnants and less-than-or-equal-to 1000 K fo r photo-dissociation regions. Hence the thermal H-2 temperature is a d iscriminant between shock excitation by supernovae and UV excitation b y OB stars. Utilizing this result, we discuss the origin of H-2 emissi on in Seyfert and starburst galaxies, which has long been controversia l.