Revisited abundance diagnostics in quasars : Fe II / Mg II ratios

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 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 α-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, IACS/Dept. of Physics, Catholic University of America. Laboratory of Astronomy and Solar Physics, NASA/Goddard Space Flight Center Greenbelt MD 20771. email: [email protected] email: [email protected] email: [email protected] Lund Observatory, Lund University, P.O. Box 43, S-22100 Lund, Sweden, [email protected] email: [email protected]