Evolution of the jet energetics revealed with the Suzaku observations of giant radio galaxies

Suzaku observations are reported of the three giant radio galaxies, 3C 35, 3C 326, and DA 240, with a projected linear size of ≳ 1 Mpc. The low and stable instrumental background of the X-ray Imaging Spectrometer onboard Suzaku allows a significant detection of inverse Compton X-ray emission from the lobes of these giant radio galaxies. As a result, the electron and magnetic energy densities in the lobes were measured. Based on the measurements, the late phase in the evolution of the energetics associated with jets from active galactic nuclei was investigated. It is found that the lobes of giant radio galaxies tend to reside in a near equipartition condition between electrons and magnetic field, despite the fact that smaller sources with sizes of about 100 kpc exhibit a particle dominance by a typical factor of 10 in energy density. In addition, the energy densities in the giant lobes are measured to be smaller by an order of magnitude as compared to the extrapolation of the size to energy density correla...