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

TL;DR

This study reveals that H_2 emission in ultra-luminous infrared galaxies originates outside the obscured star-forming regions, likely due to shocks from galaxy interactions, challenging the standard association with star formation.

Contribution

It demonstrates that H_2 emission is not directly linked to star formation but instead traces shocks caused by galaxy interactions, providing new insights into galaxy evolution.

Findings

H_2 emission arises outside obscured starburst regions

Shocks from galaxy interactions excite H_2 emission

Large-scale shocks may be common in galaxy evolution

Abstract

Ultra-luminous infrared galaxies are among the most luminous objects in the local universe and are thought to be powered by intense star formation. 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, leaving unresolved the source of excitation of this emission. Here I report an analysis of archival Spitzer Space Telescope data on ultra-luminous infrared galaxies and demonstrate that star formation regions are buried inside optically thick clouds of gas and dust, so 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 rather surprising in light of the standard view that H_2 emission is directly associated with star-formation activity. Instead, I propose that H_2 emission in these objects traces shocks in the surrounding material, which are in turn excited by interactions with nearby galaxies, and that powerful large-scale shocks cooling by means of H_2 emission may be much more common than previously thought. In the early universe, a boost in H_2 emission by this process may speed up the cooling of matter as it collapsed to form the first stars and galaxies and would make these first structures more readily observable.