Starburst in the intragroup medium of Stephan's Quintet

Based on new Infrared Space Observatory midinfrared observations and ground -based H-alpha and near-infrared observations, we report the detection of a bright starburst in the intragroup medium (IGM) of the famous compact grou p of galaxies known as Stephan's Quintet (source A in Fig. 1). We demonstra te that this starburst is caused by a collision between a high-velocity (de lta V similar to 1000 km s(-1)) intruder galaxy (NGC 7318b) 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 t o the theory for interaction-induced starbursts and may hint at a new mecha nism for the star formation excess seen in more distant clusters.