The Fe/Mg Abundance Ratio: A Diagnostic of Nucleosynthesis in the Early Universe?

Abstract

The Fe/Mg abundance ratio may be one of the fundamental indicators for nucleosynthesis in the Early Universe. Even at the highest redshift, QSO broad-lined regions (BLRs) exhibit prominent 2000-3000Å Fe II(UV) band and Mg II 2800Å resonance doublet emission in the restframe UV. The Mg is formed in Type-II SNe, while Fe has been traditionally thought to be produced in Type Ia SNe. These different origins imply a sharp falloff in Fe abundance at very high-z. However, these predictions are clouded by uncertainties about the nature of the first stars and in the nuclear yields from supernovae models. Our theoretical studies of Fe II in QSO BLRs show that Fe and Mg abundance cannot be directly deduced from the observed Fe II(UV)/Mg II, because it is sensitive to luminosity and microturbulence, as well as abundance. Observationally, support for a luminosity dependence comes from SDSS data for QSOs that show a Fe II(UV)/Mg II correlation with luminosity at z ∼ 1.8–2.0.