Resolving optical hot spots in radio galaxies with the VLT

Abstract

First detection of resolved optical emission in the hot spot regions associated with radio galaxies is presented. Very sharp images in the Js-, H-, Ks- and I-bands taken with the ESO VLT of the Southern hot spot in the radio galaxy 3C445 resolve the region in up to three point-like sources. These are embedded in diffuse optical emission, all distributed along the rims of the front shock created by the impact of the jet into the intergalactic medium. These point-like sources are identified as the places where the radio electrons transported by the jet are re-accelerated to higher energies to emit at the optical waves. Modeling of the observational data shows that relativistic electrons are accelerated by Fermi-I acceleration processes in a strong shock on a first place. However, the presence of diffuse emission and the multiple knots requires additional continuous re-acceleration which we explain via Fermi-II processes. We claim that these Fermi-II processes are powered by the development of plasma instabilities in which part of the kinetic energy of the jet is dissipated.