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

TL;DR

This study investigates optical singly-ionized iron emission in active galactic nuclei, comparing radio-quiet and radio-loud types, and explores how spectral energy distributions influence Fe II emission strength and AGN properties.

Contribution

It provides a detailed comparison of Fe II emission in different AGN types and uses photoionization models to explain observed emission differences, highlighting the unique nature of RL NLSy1s.

Findings

Fe II emission correlates with spectral types and radio properties.

Photoionization models can explain Fe II emission in radio-loud AGN.

RL NLSy1s exhibit distinct physical properties from other RL sources.

Abstract

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.