The Prevalence of Narrow Optical Fe II Emission Lines in Type 1 Active
  Galactic Nuclei
  From detailed spectral analysis of a large sample of low-redshift active
galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey, we
demonstrate---statistically for the first time---that narrow optical Fe II
emission lines, both permitted and forbidden, are prevalent in type 1 AGNs.
Remarkably, these optical lines are completely absent in type 2 AGNs, across a
wide luminosity range, from Seyfert 2 galaxies to type 2 quasars. We suggest
that the narrow FeII-emitting gas is confined to a disk-like geometry in the
innermost regions of the narrow-line region on physical scales smaller than the
obscuring torus.
