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 Å, where the prominent HI Hβ and [Oiii]λλ4959,5007 lines are also observed. We present a detailed comparison of the relative intensity of Feii 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 γ-ray detection and with prominent Feii emission. An Feii template based on I Zw 1 provides an accurate representation of the optical Feii 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 Feii emission observed in composite radio-loud spectra. However, spectral energy differences alone cannot account for the stronger Feii 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.
Highlights
The wide majority of active galactic nuclei (AGN) are characterised by the presence of broad and narrow optical and UV lines emitted by ionic species over a wide range of ionization potential χ.1 Restricting the attention to broad lines, type-1 AGN2 Spectra invariably show the same low ionization lines (χ . 20 eV) that include HI Balmer lines (Hβ, Hα), MgII λ2800, the CaII IR Triplet, and FeII features, in addition to higher ionization lines.The relative intensities of the emission lines and their profiles do not scatter around an average, but instead change in a systematic way along the so-called quasar main sequence (MS, [4,5,6,7])
The immediate result is that radio loud (RL) (CD and Fanaroff–Riley II (FR-II)) composites show weaker FeII emission [44]
This work presented an analysis of the FeII emission in the spectral bins along the main sequence where both RQ and RL coexist (A1, B1) with measurable FeII strength, and a comparison between the emission of RQ and RL composite template, along with the analysis of three RL sources found to have a stronger FeII emission
Summary
The wide majority of active galactic nuclei (AGN) are characterised by the presence of broad and narrow optical and UV lines emitted by ionic species over a wide range of ionization potential χ.1 Restricting the attention to broad lines, type-1 AGN2 Spectra invariably show the same low ionization lines (χ . 20 eV) that include HI Balmer lines (Hβ, Hα), MgII λ2800, the CaII IR Triplet, and FeII features, in addition to higher ionization lines (with CIVλ1549 as a prototypical representative in the rest-frame UV domain).The relative intensities of the emission lines and their profiles do not scatter around an average, but instead change in a systematic way along the so-called quasar main sequence (MS, [4,5,6,7]). The wide majority of active galactic nuclei (AGN) are characterised by the presence of broad and narrow optical and UV lines emitted by ionic species over a wide range of ionization potential χ.1. The MS can be represented in a plane where the full-width at half maximum (FWHM) of Hβ is diagrammed against the parameter RFeII , defined as the intensity ratio between FeII λ4570 and the broad component of Hβ that is, RFeII = F(FeII λ4570)/F(Hβ BC ). Sketches or scatter plots of the occupation of the quasar MS in this parameter plane have been often shown in the literature (e.g., [4,7,8,9]). We show a simple sketch of the quasar occupation in the optical parameter plane.
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