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

¹ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
² Faculty of Physics, University of Belgrade, Studentski Trg 12, 11000 Belgrade, Serbia
³ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski Trg 16, 11000 Belgrade, Serbia

^⋆ Corresponding author; jkovacevic@aob.bg.ac.rs

Received: 26 April 2024
Accepted: 15 January 2025

Abstract

Aims. We use a large sample of Type 1 active galactic nuclei (AGNs) spectra in order to investigate which atomic processes are responsible for some observed properties of the Fe II 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 Fe II 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 Fe II lines (Fe II_opt/Fe II_UV).

Methods. We divided the optical Fe II lines into two large line groups: consistent (Fe II_cons), whose relative intensities are in accordance with their atomic properties, and inconsistent (Fe II_incons), whose relative intensities are significantly stronger than theoretically expected. We fitted the spectra with a flexible and complex optical Fe II model, where both consistent and inconsistent Fe II lines were divided into several line groups according to their atomic characteristics and fitted independently in order to obtain more empirical clues about their properties. We focused particularly on understanding the processes that produce strong inconsistent Fe II lines, and therefore, we investigated their correlations with Fe II_cons as well as with UV Fe II lines and some measured spectral parameters.

Results. The ratios of Fe II_incons/Fe II_cons and Fe II_opt/Fe II_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 Fe II emission lines and, analogously, from the Fe II_cons to the Fe II_incons lines. In this scenario, the high Eddington ratio causes an increase in the optical depth in Fe II lines, which results in the triggering of the process of self-absorption. Measured average widths for different Fe II line groups indicate the stratification of the optical Fe II emission region. This implies that the observed Fe II spectrum is probably a complex mixture of radiation from emission regions with different physical conditions and distances from the black hole.