The Declined Activity in the Nucleus of NGC 1316

Abstract

NGC 1316 (Fornax A) is a radio galaxy with prototypical double lobes, where the magnetic field intensity is accurately measured via the inverse-Compton technique. The radio-emitting electrons in the lobes are inferred to have a synchrotron life time of 0.1 Gyr. Considering the lobe energetics, we estimate the past nuclear X-ray luminosity of NGC 1316 to be at least 4 times 10^{34} W (4 times 10^{41} erg s^{-1}). Thus, the nucleus was rather active at least 0.1 Gyr ago. In contrast, we confirmed with ASCA and ROSAT that the nucleus of NGC 1316 is very faint in X-rays at present, with the 2--10 keV luminosity of any AGN-like hard component being < 2 times 10^{33} W (2 times 10^{40} erg s^{-1}) even assuming a nuclear obscuration up to 10^{28} m^{-2} (10^{24} cm^{-2}). This is at least an order of magnitude lower than the estimated past activity, indicating that the nucleus is presently very inactive. From these two results, we conclude that the nucleus of NGC 1316 has become dormant during the last 0.1 Gyr. This suggests the possible abundance of ``dormant'' quasars in nearby galaxies.