A spectroscopic analysis of a sample of narrow-line Seyfert 1 galaxies
  selected from the Sloan Digital Sky Survey
  We revisited the spectroscopic characteristics of narrow-line Seyfert 1
galaxies (NLS1s) by analysing a homogeneous sample of 296 NLS1s at redshift
between 0.028 and 0.345, extracted from the Sloan Digital Sky Survey (SDSS-DR7)
public archive. We confirm that NLS1s are mostly characterized by Balmer lines
with Lorentzian profiles, lower black hole masses and higher Eddington ratios
than classic broad-line Seyfert 1 (BLS1s), but they also appear to be active
galactic nuclei (AGNs) contiguous with BLS1s and sharing with them common
properties. Strong Fe II emission does not seem to be a distinctive property of
NLS1s, as low values of Fe II/H$\beta$ are equally observed in these AGNs. Our
data indicate that Fe II and Ca II kinematics are consistent with the one of
H$\beta$. On the contrary, O I $\lambda$8446 seems to be systematically
narrower and it is likely emitted by gas of the broad-line region more distant
from the ionizing source and showing different physical properties. Finally,
almost all NLS1s of our sample show radial motions of the narrow-line region
highly-ionised gas. The mechanism responsible for this effect is not yet clear,
but there are hints that very fast outflows require high continuum luminosities
(> $10^{44}$ erg/s) or high Eddington ratios (log(L$_{\rm bol}$/L$_{\rm Edd}$)
> -0.1).
