A discovery of young radio sources in the cores of giant radio galaxies selected at hard X-rays

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 about 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 galaxiesselected from hard X-rayAGN catalogs selected from INTEGRAL/ IBISand Swift/BAT (thus at energies >20 keV) present a fraction of GRG four times larger than what found in radio-selected samples. We present radio observations of 15 nuclei of hard X-rayselected 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-rayemission - due to the ongoing accretion - and the reactivation of the jets.