Implications from the optical to ultraviolet flux ratio of Fe ii emission in quasars




We investigate Fe ii emission in the broad-line region (BLR) of active galactic nuclei by analysing the Fe ii(UV), Fe ii(λ4570) and Mg ii emission lines in 884 quasars in the Sloan Digital Sky Survey Quasar catalogue in a redshift range of 0.727 < z < 0.804. Fe ii(λ4570)/Fe ii(UV) is used to infer the column density of Fe ii-emitting clouds and explore the excitation mechanism of Fe ii emission lines. As suggested before in various works, the classical photoionization models fail to account for Fe ii(λ4570)/Fe ii(UV) by a factor of 10, which may suggest anisotropy of UV Fe ii emission, or an alternative mechanism like shocks. The column density distribution derived from Fe ii(λ4570)/Fe ii(UV) indicates that radiation pressure plays an important role in BLR gas dynamics. We find a positive correlation between Fe ii(λ4570)/Fe ii(UV) and the Eddington ratio. We also find that the ionizing photon fraction must be much smaller than that previously suggested unless Fe ii-emitting clouds are super-Eddington. Finally, we propose a physical interpretation of a striking set of correlations between various emission-line properties, known as ‘Eigenvector 1’.