Fe II/Mg II Emission-Line Ratio in High-Redshift Quasars

Abstract

We present results of the analysis of near-infrared spectroscopic observations of six high-redshift quasars (z>~4), emphasizing the measurement of the ultraviolet Fe II/Mg II emission-line strength to estimate the beginning of intense star formation in the early universe. To investigate the evolution of the Fe II/Mg II ratio over a wider range in cosmic time, we measured this ratio for composite quasar spectra that cover a redshift range 0<~z<~5 with nearly constant luminosity, as well as for those that span ~6 orders of magnitude in luminosity. A detailed comparison of the high-redshift quasar spectra with those of low-redshift quasars with comparable luminosity shows essentially the same Fe II/Mg II emission ratios and very similar continuum and line spectral properties, i.e., a lack of evolution of the relative iron-to-magnesium abundance of the gas in bright quasars since z~=5. Current nucleosynthesis and stellar evolution models predict that α-elements such as magnesium are produced in massive stars ending in Type II supernovae, while iron is formed predominantly in Type Ia supernovae with intermediate-mass progenitors. This results in an iron enrichment delay of ~0.2-0.6 Gyr. We conclude that intense star formation activity in the host galaxies of z>~4 quasars must have started already at an epoch corresponding to z_f~=6-9, when the age of the universe was ~0.5 Gyr (H₀=72 km s^-1 Mpc^-1, Ω_M=0.3, Ω_Λ=0.7). This epoch corresponds well to the reionization era of the universe.

Based on observations collected at the Cerro Tololo Inter-American Observatory, Chile, at the European Southern Observatory, Paranal, Chile, and the W. M. Keck Observatory, Hawaii.