Lack of Iron Abundance Evolution in High‐Redshift QSOs

We analyze the spectra of 12 high-redshift QSOs in the region of Mg II λ2798. Two six-spectrum composites, with ⟨z⟩=4.47 and 3.35, are compared with the Francis et al. Large Bright Quasar Survey (LBQS) composite (⟨z⟩~0.8) and show remarkable similarity. The equivalent width of Mg II is somewhat larger in the LBQS composite than in the high-redshift composites; this may be identified as a weak Baldwin effect. The equivalent width of the Fe II blends to either side is the same in the ⟨z⟩=3.35 composite as in the LBQS composite and marginally larger in the ⟨z⟩=4.47 composite. We derive strong confidence limits on the absence of a significant increase in either the Fe II equivalent width or the Fe II/Mg II line ratio from the earliest epoch to the present. Hamann & Ferland predicted that the Fe II/Mg II ratio at high redshift should be a factor of ~3 lower than for low-redshift QSOs if the age of the universe at the earlier epoch is much less than 1 Gyr. This is due to the delayed contribution of supernovae type Ia to the iron abundance. A 1 Gyr minimum age of the universe at z=4.47 gives interesting upper limits to q0 for a given H0 (e.g., q0<0.20 for H0=75 km s-1 Mpc-1). The lack of change in Fe II/Mg II between the two high-redshift epochs indicates that 1 Gyr may be an underestimate of the universe age at z=4.47.