What Are Narrow-Line Seyfert 1 Galaxies ?: Toward A Viewing-Angle-Dependent Unified Model for Seyfert Galaxies

Abstract

Recently growing evidence has been accumulated that the NLS1s are viewed from nearly pole-on viewing angles. However, there has been no theoretical model which explains the following important correlations; 1) weaker optical Fe II emitters always have wider emission-line widths but stronger optical Fe II emitters have either wider or narrower line widths, and 2) S1s with wider line widths always have flatter soft X-ray spectra but S1s with narrower line widths have either steeper or flatter soft X-ray spectra. Photoionization models predict that the partly-ionized zone in the broad line region (BLR) with a disk-like configuration, in which the optical Fe II emission is thought to arise, is very optically thick. If we observe this disk-like BLR from an inclined viewing angle, the Fe II emitting region located in the far-side half disk cannot be seen entirely because of the large optical depth while almost all the Hbeta emission can be seen because the ionized hydrogen is located in the outer surfaces of BLR clouds. This explains that the Fe II/Hbeta ratio depends on viewing angle. Recent discovery of relativistic outflows in some NLS1s suggests that the nuclear radio jet interacts with the dense ambient gas very close to the central engine. The kinetic Doppler effect increases the soft X-ray luminosities if we observe S1s from a nearly pole-on view given the above range of beta. These explain why NLS1s tend to have steeper soft X-ray spectra together with higher soft X-ray luminosities. Furthermore, the extinction of soft X-ray photons due to dust grain above the dusty tori are expected to more serious for the BLS1s than for the NLS1s. We thus suggest that these orientation effects are responsible for the second important correlation.