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Ultra-luminous infrared galaxies are among the most luminous objects in thelocal universe and are thought to be powered by intense star formation. It hasbeen shown that in these objects the rotational spectral lines of molecularhydrogen observed at mid-infrared wavelengths are not affected by dustobscuration, leaving unresolved the source of excitation of this emission. HereI report an analysis of archival Spitzer Space Telescope data on ultra-luminousinfrared galaxies and demonstrate that star formation regions are buried insideoptically thick clouds of gas and dust, so that dust obscuration affectsstar-formation indicators but not molecular hydrogen. I thereby establish thatthe emission of H_2 is not co-spatial with the buried starburst activity andoriginates outside the obscured regions. This is rather surprising in light ofthe standard view that H_2 emission is directly associated with star-formationactivity. Instead, I propose that H_2 emission in these objects traces shocksin the surrounding material, which are in turn excited by interactions withnearby galaxies, and that powerful large-scale shocks cooling by means of H_2emission may be much more common than previously thought. In the earlyuniverse, a boost in H_2 emission by this process may speed up the cooling ofmatter as it collapsed to form the first stars and galaxies and would makethese first structures more readily observable.

H_2 emission arises outside photodissociation regions in ultra-luminous infrared galaxies