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The issue of the difference between optical and UV properties of radio-quietand radio-loud (relativistically "jetted") active galactic nuclei (AGN) is along standing one, related to the fundamental question of why a minority ofpowerful AGN possess strong radio emission due to relativistic ejections. Thispaper examines a particular aspect: the singly-ionized iron emission in thespectral 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 relativeintensity of Fe II multiplets in the spectral types of the quasar main sequencewhere most jetted sources are found, and afterwards discuss radio-loudnarrow-line Seyfert 1 (NLSy1) nuclei with $\gamma$-ray detection and withprominent Fe II emission. An Fe II template based on I Zw 1 provides anaccurate representation of the optical Fe II emission for RQ and, with somecaveats, also for RL sources. CLOUDY photoionization simulations indicate thatthe observed spectral energy distribution can account for the modest Fe IIemission observed in composite radio-loud spectra. However, spectral energydifferences alone cannot account for the stronger Fe II emission observed inradio-quiet sources, for similar physical parameters. As for RL NLSy1s, they donot seem to behave like other RL sources, likely because of their differentphysical properties that could be ultimately associated with a higher Eddingtonratio.

Optical singly-ionized iron emission in radio-quiet and relativistically jetted active galactic nuclei