Powerful Molecular Hydrogen Line-cooling in Stephan's Quintet

Stephans Quintet (SQ) is a strongly interacting compact group experiencing a group-wide shock due to the high velocity (~ 1000 km/s) collision of an intruder galaxy with the intragroup medium. I will present recent results from deep, mid-infrared spectral mapping of SQ, using the Spitzer Space Telescope, that show for the first time the striking abundance and widespread distribution of warm H2 emission within the group. We find in SQ the phenomenal projected coexistence of hot X-ray plasma and warm H2, with the H2 emission dominating the cooling from X-ray emission. The concordance with a model of H2 emission driven by turbulent energy transfer, and the prevalence of this pathway over other sources of cooling in fast galaxy-scale shocks, may have important implications for the cooling of gas in the assembly of the first massive galaxies, as well as shock physics in systems ranging from ULIRGs to supernovae remnants.