  Galactic jets are powerful energy sources reheating the intra-cluster medium
in galaxy clusters. Their crucial role in the cosmic puzzle, motivated by
observations, has been established by a great number of numerical simulations
missing the relativistic nature of these jets. We present the first
relativistic simulations of the very long term evolution of realistic galactic
jets. Unexpectedly, our results show no buoyant bubbles, but large cocoon
regions compatible with the observed X-ray cavities. The reheating is more
efficient and faster than in previous scenarios, and it is produced by the
shock wave driven by the jet, that survives for several hundreds of Myrs.
Therefore, the X-ray cavities in clusters produced by powerful relativistic
jets would remain confined by weak shocks for extremely long periods, whose
detection could be an observational challenge.
