Searching for signatures of Fe II atomic processes in spectra of active galactic nuclei

Abstract

We use a sample of Type 1 active galactic nuclei (AGNs) spectra in order to investigate which atomic processes are responsible for some observed properties of the FeII emission lines and how they are connected with macroscopic physical characteristics of AGN emission regions. We especially focus on the violated relative intensities between different optical FeII lines, whose relative strengths do not follow the expected values according to atomic parameters. We investigated the connection between this effect and the ratio of optical to UV FeII lines (FeII$_{opt}$/FeII$_{UV}$). We divided the optical FeII lines into two large line groups: consistent (FeII$_{cons}$), whose relative intensities are in accordance with their atomic properties, and inconsistent (FeII$_{incons}$), whose relative intensities are significantly stronger than theoretically expected. We fitted the spectra with a flexible optical FeII model, where Fe II lines were divided into several line groups and fitted independently. We focused particularly on understanding the processes that produce strong inconsistent Fe II lines, and therefore, we investigated their correlations with FeII$_{cons}$ and with UV FeII lines. The ratios of FeII$_{incons}$/FeII$_{cons}$ and FeII$_{opt}$/FeII$_{UV}$ increase as the Eddington ratio increases and as the line widths decrease. It is possible that both ratios are affected by the process of self-absorption of stronger lines, which is responsible for the transmission of energy from the UV to the optical FeII emission lines and, analogously, from the FeII$_{cons}$ to the FeII$_{incons}$ lines. In this scenario, the high Eddington ratio causes an increase in the optical depth in FeII lines, which results in the triggering of the process of self-absorption. The observed FeII spectrum is probably a complex mixture of radiation from emission regions with different physical conditions.