The Unified Model of Active Galactic Nuclei: Ii. Evolutionary Connection

The classical unified model is challenged by the presence of various kinds of Seyfert 2 galaxies. We assemble a sample composed of 243 nearby Seyfert galaxies with redshifts z ≤ 0.05 to test the unification scheme. The sample contains 94 broad line Seyfert 1s (BLS1s), 44 narrow line Seyfert 1s (NLS1s), 36 X-ray absorbed hidden broad line region (BLR) Seyfert 2s (HBLR S2s), 42 X-ray absorbed non-hidden BLR Seyfert 2s (non-HBLR S2s) and 27 X-ray unabsorbed Seyfert 2s (unabsorbed non-HBLR S2s and HBLR S2s). We examine the black hole mass and accretion rate distributions in the sample. We find that: 1) NLS1s have less massive black hole masses than BLS1s; 2) HBLRS2s have the same mass distribution of the black holes with BLS1s; 3) the absorbed nonHBLR S2s have less massive black holes than HBLR S2s; 4) unabsorbed non-HBLR S2s have the most massive black holes. We thus have a queue of black hole masses from small to large: narrow to broad line Seyfert galaxies, providing new evidence for the evolutionary sequence of Seyfert galaxies. We find that the opening angles of the torus in NLS1s and absorbed non-HBLR S2s are significantly smaller than that in BLS1s and HBLR S2s. The growth of the black holes and increases of the opening angles of the tori determine the various appearances of Seyfert galaxies. We also find that the unabsorbed Seyfert 2 galaxies could be caused by low gas-to-dust ratios in the present sample. This indicates that the star formation histories could be different in the unabsorbed from in absorbed Seyfert 2 galaxies, showing evidence for suppressed star formation by black hole activities. We outline a new unification scheme based on the orientation hypothesis: Seyfert galaxies can be unified by including growth of black holes, Eddington ratios, changing opening angles of tori and gas-to-dust ratios in the tori. Seyfert galaxies are tending to finally evolve to unabsorbed non-HBLR Seyfert 2 galaxies, in which the black holes are accreting with low accretion rates and both the broad line region and dusty torus disappear. Subject headings: galaxies: active — galaxies: Seyfert — polarization — black hole: accretion — star formation