Unravelling the complex structure of the Fe II emission region in Type 1 active galactic nuclei

TL;DR

This study investigates the complex structure of Fe II emission in Type 1 AGN spectra, revealing correlations with spectral parameters and suggesting stratification and physical conditions influencing Fe II emission strength.

Contribution

It introduces a detailed decomposition of Fe II lines into groups and components, linking their behavior to the quasar main sequence and physical conditions.

Findings

Anti-correlation between Fe II EW and FWHM suggests a fundamental relation.

Strengthening of Fe II_incons and Fe II_ILR components increases Fe II emission.

Fe II_VBLR component remains relatively unchanged along the quasar main sequence.

Abstract

Using a large sample of Type 1 AGN spectra, we investigated the complex structure of the Fe II emission region in order to understand the atomic processes responsible for the enhancement of the Fe II emission. We explored correlations between Fe II features and other spectral parameters, with special focus placed on the quasar main sequence, whose underlying physics is crucial for understanding the origin of the strong Fe II emission. The Fe II emission was modelled using the flexible Fe II template that decomposes the optical Fe II lines into several line groups. According to the atomic properties of transitions, the Fe II lines were divided into inconsistent and consistent groups (Fe II$_{incons}$ and Fe II$_{cons}$), while Fe II$_{cons}$ lines were additionally decomposed into components originating from different parts of the broad-line region (Fe II$_{ILR}$ and Fe II$_{VBLR}$). We traced the behaviour of these line groups and components along the quasar main sequence. Anti-correlation between the equivalent width (EW) of Fe II and the FWHM of Fe II appears to be a more fundamental relation underlying the quasar main sequence. The increase in the EW Fe II for smaller line widths is primarily caused by the strengthening of the EW Fe II$_{incons}$ lines and, with a smaller contribution, by the enhancement of the EW Fe II$_{ILR}$ components, while the EW of Fe II$_{VBLR}$, on average, does not significantly change along the quasar main sequence. The results indicate a possible stratification of the Fe II emission region occurring in sources with strong Fe II emission. An increased Eddington ratio may modify the broad-line region structure, leading to specific physical conditions suitable for triggering additional atomic processes. This may result in the appearance of Fe II$_{incons}$ lines and FeII$_{ILR}$ components, which consequently increase the optical Fe II strength.