The Unified Model of Active Galactic Nuclei. I. Non-Hidden Broad-Line Region Seyfert 2 and Narrow-Line Seyfert 1 Galaxies

The unified model of Seyfert galaxies suggests that there are hidden broad-line regions (HBLRs) in Seyfert 2 galaxies (S2s). However, there is increasing evidence for the appearance of a subclass of S2s lacking HBLRs (non-HBLR S2s). An interesting issue arises as to the relations of non-HBLR S2s with other types of Seyfert galaxies and whether or not they can be included in the unified model. We assemble two subsamples consisting of 42 non-HBLR S2s and 44 narrow-line Seyfert 1 galaxies (NLS1s) with redshift z ≤ 0.05 from published literatures to explore this issue. We compare black hole masses in the galactic centers, accretion rates, and infrared color ratio (f60 μm/f25 μm) as a potential indicator of the dusty torus orientation, [O III] λ5007, radio, and far-infrared luminosities. We find that non-HBLR S2s and NLS1s have (1) similar distributions of black hole masses [ × 107 M☉] and Eddington ratios (LBol/LEdd ~ 1); (2) significantly different distributions of f60 μm/f25 μm ratios; and (3) similar distributions of bulge magnitudes and luminosities of [O III], radio, and far-infrared emission. The similarities and differences can be understood naturally if they are intrinsically the same, but non-HBLR S2s are viewed at larger angles of the observer's sight than NLS1s. We thus suggest that non-HBLR S2s only have "narrower" broad-line regions and that they are the counterparts of NLS1s viewed at high inclination angles. The absence of the polarized emission line in non-HBLR S2s is caused by the less massive black holes and high accretion rate similar to NLS1s. The implications of the unification scheme of non-HBLR S2s and NLS1s are discussed.