The Formation of FeII Emission Lines

Abstract

The excitation of FeII emission lines is reviewed, with special emphasis on photoionized nebulae. The physical conditions, the geometry and the techniques required to calculate FeII models are explained and the following mechanisms discussed: a: Excitation by absorption of continuum radiation. b: Excitation by inelastic collisions, and c: Excitation by line fluorescence. Possibility a. has been over-looked in the past and is probably not important in objects showing strong optical FeII lines. Collisional excitation dominates in nature but cannot, in itself, explain the strong observed lines from energy levels of 7 to 10 eV. Line fluorescence, with L_α, CIV and most important among FeII lines, is probably the key to the understanding of the high energy lines. A recipe for calculating FeII models is given, showing the processes to include and the techniques to use. Major recent findings about the FeII spectrum of AGN are discussed in detail, in particular “the FeII problem” in these objects. Despite big improvements in the understanding of the 2000–4000A emission bump and other FeII features, there are still several open questions like the role of dust, the iron abundance, the geometry and density of the broad line region and the general validity of photoionization models.