Optical singly-ionized iron emission in radio-quiet and relativistically
  jetted active galactic nuclei
  The issue of the difference between optical and UV properties of radio-quiet
and radio-loud (relativistically "jetted") active galactic nuclei (AGN) is a
long standing one, related to the fundamental question of why a minority of
powerful AGN possess strong radio emission due to relativistic ejections. This
paper examines a particular aspect: the singly-ionized iron emission in the
spectral range 4400 -- 5600 A, where the prominent HI H$\beta$ and [OIII] 4959,
5007 lines are also observed. We present a detailed comparison of the relative
intensity of Fe II multiplets in the spectral types of the quasar main sequence
where most jetted sources are found, and afterwards discuss radio-loud
narrow-line Seyfert 1 (NLSy1) nuclei with $\gamma$-ray detection and with
prominent Fe II emission. An Fe II template based on I Zw 1 provides an
accurate representation of the optical Fe II emission for RQ and, with some
caveats, also for RL sources. CLOUDY photoionization simulations indicate that
the observed spectral energy distribution can account for the modest Fe II
emission observed in composite radio-loud spectra. However, spectral energy
differences alone cannot account for the stronger Fe II emission observed in
radio-quiet sources, for similar physical parameters. As for RL NLSy1s, they do
not seem to behave like other RL sources, likely because of their different
physical properties that could be ultimately associated with a higher Eddington
ratio.
