Fe II Diagnostic Tools for Quasars

Abstract:
The enrichment of Fe relative to α-elements such as O and Mgrepresents a potential means to determine the age of quasars and probethe galaxy formation epoch. To explore how Fe II emission in quasars islinked to physical conditions and abundance, we have constructed an 830level Fe II model atom and investigated through photoionizationcalculations how Fe II emission strengths depend on nonabundancefactors. 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 thebroad-line regions (BLRs) of quasars. Our calculations show that (1)similar Fe II (UV)/Mg II ratios can exist over a wide range of physicalconditions, (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 fluxprovide tools to derive distances to BLRs in quasars. To derive all BLRphysical parameters with uncertainties, comparisons of our model withobservations of a large quasar sample at low redshift (z<1) aredesirable. The STIS and NICMOS instruments aboard the Hubble SpaceTelescope offer the best means to provide such observations.