Elucidating the Correlation of the Quasar Fe II/Mg II Ratio with Redshift

Abstract

Interpretation of the Fe II(UV)/Mg II emission ratios from quasars has a major cosmological motivation. Both Fe and Mg are produced by short-lived massive stars. In addition, Fe is produced by accreting white dwarf supernovae somewhat after star formation begins. Therefore, we expect that the Fe/Mg ratio will gradually decrease with redshift. We have used data from the Sloan Digital Sky Survey to explore the dependence of the Fe II(UV)/ Mg II ratio on redshift and on luminosity in the redshift range of 0.75 < z < 2.20, and we have used predictions from our 830-level model for the Fe II atom in photoionization calculations to interpret our findings. We have split the quasars into several groups based on the value of their Fe II(UV)/Mg II emission ratios and then checked to see how the fraction of quasars in each group varies with the increase of redshift. We next examined the luminosity dependence of the Fe II(UV)/Mg II ratio, and we found that beyond a threshold of Fe II(UV)/Mg II = 5, and M2500 < -25 mag, the Fe II(UV)/Mg II ratio increases with luminosity, as predicted by our model. We interpret our observed variation of the Fe II(UV)/Mg II ratio with redshift as a result of the correlation of redshift with luminosity in a magnitude-limited quasar sample.