Narrow-line Seyfert 1 galaxies in the context of the Quasar Main
  Sequence
  Narrow-line Seyfert 1 galaxies are defined on the basis of their line widths,
and they are generally considered to be high Eddington ratio sources. But in
the context of the Quasar Main Sequence, high Eddington rate sources are those
which have weak [O III] lines and strong Fe II lines. There is an overlap
between the two populations, but they are not identical. Thus these two
selection criteria give a different view on which objects are actually high
Eddington ratio sources. We discuss this issue in the context of the broad band
spectral energy density, emission line shape modeling, Fe II pseudo-continuum
strength, and the level of X-ray variability. We also discuss the issue of the
viewing angle and the insight one can gain from spectropolarimetric
observations. We conclude that there is no single driver behind the Quasar Main
Sequence, and the Eddington ratio of the source cannot be determined from the
source location on the optical plane alone. On the other hand, an expected
range of AGN parameters combined with a simple model of the Fe II production
represent well the observed coverage pattern of the plane, with not much effect
needed from the dispersion due to the viewing angle.
