The Declined Activity in the Nucleus of NGC 1316

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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 lifetime of ~0.1 Gyr. Considering the lobe energetics, we estimate the past nuclear X-ray luminosity of NGC 1316 to be at least ~4×1034 W (4×1041 ergs 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 active galactic nucleus-like hard component being less than 2×1033 W (2×1040 ergs s-1), even assuming a nuclear obscuration up to ~1028 m-2 (1024 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.
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