Analysis of Optical Fe II Emission in a Sample of Active Galactic Nucleus Spectra

International audienceWe present a study of optical Fe II emission in 302 active galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey. We group the strongest Fe II multiplets into three groups according to the lower term of the transition (b4 F, a6 S, and a4 G terms). These approximately correspond to the blue, central, and red parts, respectively, of the "iron shelf" around Hbeta. We calculate an Fe II template that 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, Halpha, Hbeta, [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 note 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 Hbeta, the velocity shift of Fe II, and the Halpha/Hbeta 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 & Green Eigenvector 1 (EV1), this implies a connection between the Baldwin effect and EV1 and could be connected with AGN evolution. We find that spectra are different for Hbeta FWHMs greater and less than ~3000 km s-1, and that there are different correlation coefficients between the parameters