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

Using a large sample of Type 1 active galactic nucleus spectra, we investigated the complex structure of the Fe II emission region with the aim of understanding 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 full width at half maximum 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 the formation of localised regions with specific physical conditions suitable for triggering additional atomic processes. This may result in the appearance of Fe II_incons lines and Fe II_ILR components, which consequently increase the optical Fe II strength.