A Discovery of Young Radio Sources in the Cores of Giant Radio Galaxies Selected at Hard X-Rays

Abstract

Giant radio galaxies (GRG) are the largest single entities in the universe, having a projected linear size exceeding 0.7 Mpc, which implies that they are also quite old objects. They are not common, representing a fraction of only ∼6% in samples of bright radio galaxies. While a census of about 300 of these objects has been built in the past years, still no light has been shed on the conditions necessary to allow such an exceptional growth, whether of environmental nature or linked to the inner accretion properties. Recent studies found that samples of radio galaxies selected from hard X-ray active galactic nuclei catalogs selected from the International Gamma-Ray Astrophysics Laboratory (INTEGRAL)/the Imager on Board the INTEGRAL Satellite and Swift/the Burst Alert Telescope (thus at energies >20 keV) present a fraction of GRG four times larger than what is found in radio-selected samples. We present radio observations of 15 nuclei of hard-X-ray-selected GRG, finding for the first time a large fraction (61%) of young radio sources at the center of Mpc-scale structures. Being at the center of GRG, these young nuclei may be undergoing a restarting activity episode, suggesting a link between the detected hard X-ray emission—due to the ongoing accretion—and the reactivation of the jets.