The line parameters and ratios as the physical probe of the line emitting regions in AGN

TL;DR

This paper investigates the physical conditions of emission line regions in AGN, using line fluxes to probe the properties of the BLR and Fe II emission, revealing that Fe II originates from an intermediate line region.

Contribution

It introduces a method to use line fluxes to analyze the physical conditions of AGN emission regions and identifies the Fe II emission as coming from an intermediate line region.

Findings

Hydrogen Balmer and helium lines help estimate BLR parameters.

Fe II emission mainly originates from an intermediate line region.

Fe II region is located further from the central source than the BLR.

Abstract

Here we discuss the physical conditions in the emission line regions (ELR) of active galactic nuclei (AGN), with the special emphasize on the unresolved problems, e.g. the stratification of the Broad Line Region (BLR) or the failure of the photoionization to explain the strong observed optical Fe II emission. We use here different line fluxes in order to probe the properties of the ELR, such as the hydrogen Balmer lines (Ha to He), the helium lines from two subsequent ionization levels (He II 4686 and He I 5876) and the strongest Fe II lines in the wavelength interval 4400-5400 \AA. We found that the hydrogen Balmer and helium lines can be used for the estimates of the physical parameters of the BLR, and we show that the Fe II emission is mostly emitted from an intermediate line region (ILR), that is located further away from the central continuum source than the BLR.