Starburst in the Intragroup Medium of Stephan's Quintet

Based on observations made with ISO, an ESA project with instruments funded by ESA Member States and with the participation of ISAS and NASA. – 2 – Based on new ISO mid-infrared observations and ground based Hα and near-infrared observations, we report the detection of a bright starburst in the intragroup medium (IGM) of the famous compact group of galaxies Stephan’s Quintet (Source A in Fig.1). We demonstrate that this starburst is caused by a collision between a high velocity (δV ∼ 1000 km/sec) intruder galaxy (NGC7318b) and the IGM of the group. While this is the only starburst known today that is induced by a galaxy/cold-intergalactic-medium collision, it provides new constraints to the theory for interaction-induced starbursts, and may hint at a new mechanism for the star formation excess seen in more distant clusters.