Are Narrow Line Seyfert 1s a special class of Active Galactic Nuclei?
  No. Due to their apparently extreme optical to X-ray properties, Narrow Line
Seyfert 1s (NLSy1s) have been considered a special class of active galactic
nuclei (AGN). Here, we summarize observational results from different groups to
conclude that none of the characteristics that are typically used to define the
NLSy1s as a distinct group - from the, nowadays called, Broad Line Seyfert 1s
(BLSy1s) - is unique, nor ubiquitous of these particular sources, but shared by
the whole Type 1 AGN. Historically, the NLSy1s have been distinguished from the
BLSy1s by the narrow width of the broad Hbeta emission line. The upper limit on
the full width at half maximum of this line is 2000 km/s for NLSy1s, while in
BLSy1s it can be of several thousands of km/s. However, this border has been
arbitrarily set and does not correspond to the change of any physical property.
All observed parameters in Type 1 AGN cover continues ranges of values, which
does not allow to infer the existence of two different kind of populations with
FWHM(Hbeta,broad) = 2000 km/s as dividing point. We argue that the usage of
this velocity limit to define samples of NLSy1s galaxies - as it is usually
done in comparative studies -, together with the well known observational
biases, naturally favors the selection of sources with low black hole masses
and high Eddington ratios that are hosted by blue spiral galaxies. Therefore
selection biases might be responsible for the reported differences between
NLSy1 and BLSy1 sources.
