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

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Abstract

Both the Fe ii UV emission in the 2000-3000 A region [Fe ii(UV)] and resonance emission-line complex of Mg ii at 2800 A are prominent features in quasar spectra. The observed Fe ii(UV)/Mg ii emission ratios have been proposed as means to measure the buildup of the Fe abundance relative to that of the a-elements C, N, O, Ne, and Mg as a function of redshift. The current observed ratios show large scatter and no obvious dependence on redshift. Thus, it remains unresolved whether a dependence on redshift exists and whether the observed Fe ii(UV)/Mg ii ratios represent a real nucleosynthesis diagnostic. We have used our new 830 level model atom for Fe in photoionization calculations, reproducing the physical conditions in the broad-line regions of quasars. This modeling reveals that interpretations of high values of Fe ii(UV)/Mg ii are sensitive not only to Fe and Mg abundance, but also to other factors such as microturbulence, density, and properties of the radiation field. We find that 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 4000-6000 A band, can be used as a reliable nucleosynthesis diagnostic for the Fe/Mg abundance ratios for the physical conditions relevant to the broad-line regions of quasars. This has extreme importance for quasar observations with the Hubble Space Telescope and also with the future James Webb Space Telescope.

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