Spitzer Observations of Stephan's Quintet -- IGM Dust and Gas in a Multi-galaxy Collision

Stephan's Quintet (SQ) is the most famous and well studied compact group of galaxies. Spectacular activity is triggered by interactions between member galaxies and various constituents of the intragroup medium (IGM), including a ~40 kpc large scale-shock and IGM starbursts more than 20 kpc away from any neighboring galaxy centers. Our new Spitzer IRS (SH, LH, and SL) and MIPS (70μ m and 160 μ m) observations of SQ with unprecedented resolution and sensitivity give new constraints on the physical conditions of the IGM gas and dust. In particular, our IRS observations discovered very strong pure MIR emission lines of molecular hydrogen in the SQ shock, the first time such a spectrum is detected in an extragalactic source. And the MIPS 160μ m map of SQ shows clear evidence for the diffuse IGM dust emission.