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

Abstract

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 of HBLR (non-HBLR S2s). An interesting issue arises as to 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 sub-samples consisting of 42 non-HBLR S2s and 44 narrow-line Seyfert 1s (NLS1s) with redshift $z\le 0.05$ from published literatures to explore this issue. We compare black hole masses in the galactic centers, accretion rates, infrared color ratio ($f_{60 μ\rm m}/f_{25 μ\rm m}$) as a potential indicator of the dusty torus orientation, \oiii $λ5007$, radio and far infrared luminosities. We find that non-HBLR S2s and NLS1s have: 1) similar distributions of the black hole masses ($10^6-3\times 10^7\sunm$) and the Eddington ratios ($L_{\rm Bol}/L_{\rm Edd}\sim 1$); 2) significantly different distributions of $f_{60 μ\rm m}/f_{25 μ\rm m}$ ratios; 3) similar distributions of bulge magnitudes and luminosities of [O {\sc iii}], radio, far infrared emission. The similarities and differences can be understood naturally if they are intrinsically same but non-HBLR S2s are viewed at larger angles of observer's sight than NLS1s. We thus suggest that non-HBLR S2s only have "narrower" broad line regions and 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.