Starburst in the Intragroup Medium of Stephan's Quintet

Abstract

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 group of galaxies known as Stephan's Quintet (source A in Fig. 1). We demonstrate that this starburst is caused by a collision between a high-velocity (deltaV~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 to the theory for interaction-induced starbursts and may hint at a new mechanism for the star formation excess seen in more distant clusters.