Analysis of optical Fe II emission in a sample of AGN spectra

TL;DR

This study analyzes optical Fe II emission in 302 AGNs, developing a new template for better spectral fitting, revealing correlations with other emission lines, and exploring implications for AGN evolution and the Eigenvector 1 parameter.

Contribution

A new Fe II emission template based on I Zw 1 improves spectral fits and reveals correlations with AGN properties, advancing understanding of Fe II origin and AGN evolution.

Findings

Fe II likely originates in an Intermediate Line Region.

Different parts of the iron shelf vary with AGN properties.

Inverse Baldwin effect observed for Fe II emission.

Abstract

We present a study of optical Fe II emission in 302 AGNs selected from the SDSS. We group the strongest Fe II multiplets into three groups according to the lower term of the transition (b $^4$F, a $^6$S and a $^4$G terms). These correspond approximately to the blue, central, and red part respectively of the "iron shelf" around Hb. We calculate an Fe II template which takes into account transitions into these three terms and an additional group of lines, based on a reconstruction of the spectrum of I Zw 1. This Fe II template gives a more precise fit of the Fe II lines in broad-line AGNs than other templates. We extract Fe II, Ha, Hb, [O III] and [N II] emission parameters and investigate correlations between them. We find that Fe II lines probably originate in an Intermediate Line Region. We notice that the blue, red, and central parts of the iron shelf have different relative intensities in different objects. Their ratios depend on continuum luminosity, FWHM Hb, the velocity shift of Fe II, and the Ha/Hb flux ratio. We examine the dependence of the well-known anti-correlation between the equivalent widths of Fe II and [O III] on continuum luminosity. We find that there is a Baldwin effect for [O III] but an inverse Baldwin effect for the Fe II emission. The [O III]/Fe II ratio thus decreases with L\lambda5100. Since the ratio is a major component of the Boroson and Green eigenvector 1, this implies a connection between the Baldwin effect and eigenvector 1, and could be connected with AGN evolution. We find that spectra are different for Hb FWHMs greater and less than ~3000 km/s, and that there are different correlation coefficients between the parameters.