Relativistic and slowing down: The flow in the hot spots of powerful radio galaxies and quasars

Pairs of radio-emitting jets with lengths up to several hundred kiloparsecs emanate from the central region ( the "core") of radio-loud active galaxies. In the most powerful of them, these jets terminate in the "hot spots," compact high-brightness regions, where the jet flow collides with the intergalactic medium (IGM). Although it has long been established that in their inner (similar to1 pc) regions these jet flows are relativistic, it is still not clear if they remain so at their largest ( hundreds of kiloparsecs) scales. We argue that the X-ray, optical, and radio data of the hot spots, despite their at-first-sight disparate properties, can be unified in a scheme involving a relativistic flow upstream of the hot spot that decelerates to the subrelativistic speed of its inferred advance through the IGM and viewed at different angles to its direction of motion. This scheme, besides providing an account of the hot spot spectral properties with jet orientation, also suggests that the large-scale jets remain relativistic all the way to the hot spots.