H2 emission arises outside photodissociation regions in ultraluminous infrared galaxies

Ultraluminous infrared galaxies are among the most luminous objects in the local Universe and are thought to be powered by intense star formation(1,2). It has been shown that in these objects the rotational spectral lines of molecular hydrogen observed at mid-infrared wavelengths are not affected by dust obscuration(3), but left unresolved was the source of excitation for this emission. Here I report an analysis of archival Spitzer Space Telescope data on ultraluminous infrared galaxies and demonstrate that dust obscuration affects star formation indicators but not molecular hydrogen. I thereby establish that the emission of H-2 is not co-spatial with the buried starburst activity and originates outside the obscured regions. This is unexpected in light of the standard view that H-2 emission is directly associated with star-formation activity(3-5). I propose the alternative view that H-2 emission in these objects traces shocks in the surrounding material that are excited by interactions with nearby galaxies. Large-scale shocks cooling by means of H-2 emission may accordingly be more common than previously thought. In the early Universe, a boost in H-2 emission by this process may have accelerated the cooling of matter as it collapsed to form the first stars and galaxies, and would make these first structures more readily observable(6).