Fe II Diagnostic Tools for Quasars

The enrichment of Fe relative to α-elements such as O and Mg represents a potential means to determine the age of quasars and probe the galaxy formation epoch. To explore how Fe II emission in quasars is linked to physical conditions and abundance, we have constructed an 830 level Fe II model atom and investigated through photoionization calculations how Fe II emission strengths depend on nonabundance factors. We have split Fe II emission into three major wavelength bands, Fe II (UV), Fe II (Opt1), and Fe II (Opt2), and explore how the Fe II (UV)/Mg II, Fe II (UV)/Fe II (Opt1), and Fe II (UV)/Fe II (Opt2) emission ratios depend on hydrogen density and ionizing flux in the broad-line regions (BLRs) of quasars. Our calculations show that (1) similar Fe II (UV)/Mg II ratios can exist over a wide range of physical conditions, (2) the Fe II (UV)/Fe II (Opt1) and Fe II (UV)/Fe II (Opt2) ratios serve to constrain ionizing luminosity and hydrogen density, and (3) flux measurements of Fe II bands and knowledge of the ionizing flux provide tools to derive distances to BLRs in quasars. To derive all BLR physical parameters with uncertainties, comparisons of our model with observations of a large quasar sample at low redshift (z < 1) are desirable. The STIS and NICMOS instruments aboard the Hubble Space Telescope offer the best means to provide such observations.