Evidence for a Dramatic Activity Decline in the Nucleus of the Radio Galaxy Fornax A

Fornax A (NGC 1316) is a radio galaxy with prototypical double lobes. Feigelson et al. (1995 ApJ 449, L149) and Kaneda et al. (1995 ApJ 454, L13) detected inverse Compton X-rays for the first time from its radio lobes, and unambiguously derived the lobe magnetic field intensity of 2-4 μG. Accordingly, the radio-emitting electrons in the lobes are inferred to have a Lorentz factor of 104, and hence a synchrotron life time of ∼ 108 yr. This means that Fornax A was highly active at least 108 years ago.