The Unified Model of Active Galactic Nuclei. II. Evolutionary Connection

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 emission line Seyfert 1 galaxies (BLS1s), 44 narrow emission line Seyfert 1s (NLS1s), 36 X-ray-absorbed hidden broad line region Seyfert 2s (HBLR S2s), 42 X-ray-absorbed non-HBLR S2s, and 27 X-ray-unabsorbed Seyfert 2s (unabsorbed non-HBLR S2s and HBLR S2s). We find that (1) NLS1s have less massive black hole masses than BLS1s, (2) HBLR S2s have the same mass distribution of black holes as BLS1s, (3) absorbed non-HBLR S2s have less massive black holes than HBLR S2s, and (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. We find that the opening angles of the torus in NLS1s and absorbed non-HBLR S2s are significantly smaller than those in BLS1s and HBLR S2s. 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 and 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 the growth of black holes, Eddington ratios, changing opening angles of tori, and gas-to-dust ratios in tori. Seyfert galaxies tend to finally evolve to unabsorbed non-HBLR Seyfert 2 galaxies, in which the black holes accrete with low accretion rates and both the broad-line region and dusty torus disappear.