A high-resolution look at dust emission from shocks and star-forming regions in Stephan.s Quintet

Stephan.s Quintet (SQ) is a compact group of galaxies, where tidal interactions have brought large amounts of gas into the intergalactic medium (IGM). Spitzer IRS observations revealed an unusually bright H2 line emission from a giant (40 kpc long), Xray emitting shock attributed to a highspeed (1000 km/s) galaxy collision. The extreme H2toPAH flux ratio (1000 times more than starforming galaxies) suggest that H2 is predominantly excited by shocks rather than UV heating from starforming regions. From our OT1 P2 project, we recently obtained SPIRE images of SQ, revealing for the first time the cold dust associated with the warm H2 gas in the shock. However, the large SPIRE beams do not separate starforming regions from shocked gas. To confirm what fraction of the dust emission comes from the shock, we propose PACS observations with higher angular resolution. These observations are further motivated by our recent PdBI CO(10) observations, which break the SQ shock in large complexes (35 kpc in size) with a range of line widths (40200km/s). Some of them could be sites of star formation. PACS is the only instrument providing the sensitivity and angular resolution to quantify star formation on these scales. With the proposed observations, for the first time for IGM gas, we will be able to (1) calibrate the CO to H2 conversion factor using dust emission and estimate the total cold gas content of the CO complexes, (2) study the impact of turbulence on starformation efficiency, and (3) constrain the dust size distribution. The results will have important consequences on our understanding of the energetics and the role of dust in cooling the IGM gas in highredshift mergers, and in the formation of galaxies. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]