The Declined Activity in the Nucleus of NGC 1316

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 × 10 W (4 × 10 erg s). 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 × 10 W (2 × 10 erg s) even assuming a nuclear obscuration up to ∼ 10 m (10 cm). 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. Subject headings: galaxies: active — galaxies: jet — galaxies: nuclei — galaxies: individual (NGC 1316) — X-rays: galaxies 1Department of Physics, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 2Research Center for the Early Universe, University of Tokyo, 7-3-1 Hongo, Tokyo, Japan 113-0033 3Department of Physics, School of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo, Japan 192-0364 4The Institute of the Space and Astronautical Science, Yoshinodai, Sagamihara, Kanagawa, Japan 229-0022