Powerful High-Velocity Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's Quintet

We present the discovery of strong mid-infrared emission lines of molecularhydrogen of apparently high velocity dispersion (~870 km/s) originating from agroup-wide shock wave in Stephan's Quintet. These Spitzer Space Telescopeobservations reveal emission lines of molecular hydrogen and little else. thisis the first time an almost pure H_2 line spectrum has been seen in anextragalactic object. Along with the absence of PAH features and very lowexcitation ionized gas tracers, the spectra resemble shocked gas seen inGalactic supernova remnants, but on a vast scale. The molecular emissionextends over 24 kpc along the X-ray emitting shock-front, but has ten times thesurface luminosity as the soft X-rays, and about one-third the surfaceluminosity of the IR continuum. We suggest that the powerful H_2 emission isgenerated by the shock wave caused when a high-velocity intruder galaxycollides with filaments of gas in the galaxy group. Our observations suggest aclose connection between galaxy-scale shock-waves and strong broad H_2 emissionlines, like those seen in the spectra of Ultraluminous Infrared Galaxies wherehigh-speed collisions between galaxy disks are common.Comment: 4 pages, 4 figures and 1 tabl