Narrow-line Seyfert 1 galaxies ( NLS1s ) have been recognized to comprise a distinct class of the Seyfert activity in addition to type 1 and type 2 Seyfert galaxies ( S1s and S2s , respectively ) ; the NLS1s show strong optical Fe ii emission lines as well as high-ionization lines without optical broad emission lines .
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 ; e.g. , the hydrogen column density is required to be as high as N _ { H } \gtrsim 10 ^ { 24.5 } cm ^ { -2 } .
Therefore , the visibility of the partly-ionized regions is expected to be strongly viewing-angle dependent .
If we observe this disk-like BLR from an inclined viewing angle ; e.g. , i _ { view } \simeq 30 ^ { \circ } [ a typical viewing angle for broad-line S1s ( hereafter BLS1s ) ] , the Fe ii emitting region located in the far-side half disk can not be seen entirely because of the large optical depth while almost all the H \beta emission can be seen because the ionized hydrogen is located in the outer surfaces of BLR clouds .
On the other hand , if we observe the disk-like BLR from a nearly pole-on view , we can see both Fe ii and H \beta , resulting in a higher Fe ii /H \beta ratio together with a narrower line width with respect to those observed from an inclined viewing angle .
This explains the first important correlation .

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 .
We show that this can be responsible for the formation of hot plasma with kinetic temperatures of T _ { kin } \sim 10 ^ { 6 } K , giving rise to the production of soft X-ray photons .
The black-body radiation from the hot plasma explains the observed steeper photon indices in the soft X ray if T _ { kin } \simeq ( 1 – 2 ) \times 10 ^ { 6 } K and \beta = v _ { jet } / c \simeq 0.2 – 0.7 .
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 .
Since the pole-on view model for NLS1s implies that BLS1s are viewed from intermediate orientations , the brightening due to the kinematic Doppler effect is generally weak for the BLS1s and thus the soft X-ray excess emission in the BLS1s is less dominant than that in the NLS1s .
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 .
It is also understood why soft X-ray surveys tend to pick up NLS1s preferentially .

It is known that there is a correlation between the soft X-ray photon index ( \Gamma ) and the Fe ii \lambda 4570/H \beta ratio for both NLS1 and BLS1s .
We also newly find another correlation between \Gamma and the soft X-ray luminosity for the NLS1s studied by Boller , Brandt , & Fink .
These correlations suggest that the strength of Fe ii emission is intimately related to the soft X-ray emission .
Since the ultraviolet continuum emission from the optically-thick accretion disk with T \sim 10 ^ { 5 } K can not create partly-ionized regions in the disk-like BLR efficiently , we suggest that soft X-ray photons from the jet-driven shocked regions with T \sim 10 ^ { 6 } K are responsible for the formation of the Fe ii emitting regions .
This provides a causal relationship between the strong Fe ii emission and the excess soft X-ray emission .

Finally , we propose a viewing-angle-dependent unified model for Seyfert nuclei ; 1 ) 0 ^ { \circ } \leq i _ { view } \lesssim 10 ^ { \circ } for NLS1s , 2 ) 10 ^ { \circ } \lesssim i _ { view } \lesssim 30 ^ { \circ } for BLS1s , 3 ) 30 ^ { \circ } \lesssim i _ { view } \lesssim 50 ^ { \circ } for type 2 Seyferts with the hidden BLR ( S2 ^ { + } s ) , and 4 ) 50 ^ { \circ } \lesssim i _ { view } \leq 90 ^ { \circ } for type 2 Seyferts without the hidden BLR ( S2 ^ { - } s ) where i _ { view } is the viewing angle toward the BLR disk around the central engine .
Since the NLS1 phenomenon is also observed in radio-quiet quasars , it is strongly suggested that the class of NLS1s is the radio-quiet equivalent of the class of Blazers in radio-loud active galactic nuclei .