Revisited Abundance Diagnostics in Quasars: Fe II/Mg II Ratios

Abstract:
Both the Fe II UV emission in the 2000-3000 Å region [Fe II(UV)]and resonance emission line complex of Mg II at 2800 Å areprominent features in quasar spectra. The observed Fe II(UV)/Mg IIemission ratios have been proposed as means to measure the buildup ofthe Fe abundance relative to that of the alpha-elements C, N, O, Ne,and Mg as a function of redshift. The current observed ratios show largescatter and no obvious dependence on redshift. Thus, it remainsunresolved whether a dependence on redshift exists and whether theobserved Fe II(UV)/Mg II ratios represent a real nucleosynthesisdiagnostic. We have used our new 830 level model atom for Fe<SUP>+</SUP>in photoionization calculations, reproducing the physical conditions inthe broad-line regions of quasars. This modeling reveals thatinterpretations of high values of Fe II(UV)/Mg II are sensitive not onlyto Fe and Mg abundance, but also to other factors such asmicroturbulence, density, and properties of the radiation field. We findthat the Fe II(UV)/Mg II ratio combined with Fe II(UV)/Fe II(optical)emission ratio, where Fe II(optical) denotes Fe II emission in 4000-6000Å band, can be used as a reliable nucleosynthesis diagnostic forthe Fe/Mg abundance ratios for the physical conditions relevant to thebroad-line regions of quasars. This has extreme importance for quasarobservations with the Hubble Space Telescope and also with the futureJames Webb Space Telescope.