Reenergization of Radio Halo Electrons in the Merging Galaxy Cluster A2163

Abstract

Radio halos in galaxy clusters are among the largest known radio-emitting objects in the universe. Their faint and amorphous structure and lack of emission elsewhere in the electromagnetic spectrum have yielded few clues to their origin. The strong association of halos with X-ray-luminous merging clusters has sustained the belief that mergers kindle halos. We report the detection of a sharply defined, central Ridge of halo emission in the galaxy cluster A2163, which is undergoing a merger. This "shock front"-like Ridge lies in between and is oriented perpendicular to the two merging subclusters, and is not associated with any active radio galaxy. This Ridge has the flattest spectrum and the brightest emission in the entire halo, suggesting that it is the principal site of energization, and appears to be a direct link between the merger geometry and halo energization. We also discuss the possibility that the Ridge is a relic seen in projection against the center of tthe halo. However, the configuration of the Ridge relative to the merging subclusters suggests that it is at the center of the halo. Based on our observations we suggest that a halo may simply be the merger-induced collective reincarnation of a large number of dead radio lobes.