New insights into the evolution of the FR I radio galaxy 3C 270 (NGC 4261) from VLA and GMRT radio observations

We present the Giant Metrewave Radio Telescope 240 MHz observations of the nearby luminous Fanaroff-Riley type I radio source 3C 270, in the group-central elliptical NGC 4261. Combining these data with reprocessed Very Large Array 1.55 and 4.8 GHz observations, we produce spectral-index maps that reveal a constant spectral index along the jets and a gradual steepening from the ends of the jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted to the integrated spectrum of the source gives an asymptotic low-frequency index of α<inf>inj</inf> = 0.53^+0.01<inf>-0.02</inf>, while JP models fitted to the observed spectral index trend along the lobes allow us to estimate radiative ages of ~29 and ~37 Myr for the west and east lobes, respectively. Our age estimates are a factor of 2 lower than the 75-Myr upper limit derived from X-ray data. We find unlikely the scenario of an early supersonic phase in which the lobe expanded into the ISM at approximately Mach 6 (3500 km s^-1), and suggest that either the source underwent multiple active galactic nuclei outbursts with possible large changes in jet power, or possibly that the source age that we find is due to a backflow that transports young electrons from the jet tips through the lobes towards the nucleus relatively quickly. We calculate that in the lobes the energy ratio of non-radiating to radiating particles is ~4-24 indicating significant gas entrainment. If the lobes are in pressure balance with their surroundings, the total energy required to heat the entrained material is 10⁵⁸ erg,~40 per cent of the total enthalpy of the lobes.