FeII/MgII emission-line ratio in high-redshift quasars

We present results of the analysis of near-infrared spectroscopic observations of six high-redshift quasars (z greater than or similar to 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 greater than or similar to z less than or similar to 5 with nearly constant luminosity, as well as for those that span similar to6 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 similar or equal to 5. Current nucleosynthesis and stellar evolution models predict that alpha-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 similar to 0.2-0.6 Gyr. We conclude that intense star formation activity in the host galaxies of z greater than or similar to 4 quasars must have started already at an epoch corresponding to z(f) similar or equal to 6-9, when the age of the universe was similar to0.5 Gyr (H-0 = 72 km s(-1) Mpc(-1), Omega(M) = 0.3, Omega(Lambda) = 0.7). This epoch corresponds well to the reionization era of the universe.