We present results of the analysis of near infrared spectroscopic observations of 6 high-redshift quasars ( z \gtrsim 4 ) , emphasizing the measurement of the ultraviolet Fe ii / Mg ii emission line strength in order 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 which cover a redshift range of 0 \lesssim z \lesssim 5 with nearly constant luminosity , as well as for those which span \sim 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 \simeq 5 .
Current nucleosynthesis and stellar evolution models predict that \alpha -elements like magnesium are produced in massive stars ending in type II SNe , while iron is formed predominantly in SNe of type Ia with intermediate mass progenitors .
This results in an iron enrichment delay of \sim 0.2 { to } 0.6 Gyr .
We conclude that intense star formation activity in the host galaxies of z \gtrsim 4 quasars must have started already at an epoch corresponding to z _ { f } \simeq 6 - 9 , when the age of the universe was \sim 0.5 Gyr ( H _ { o } = 72 km s ^ { -1 } Mpc ^ { -1 } , \Omega _ { M } = 0.3 , \Omega _ { \Lambda } = 0.7 ) .
This epoch corresponds well to the re-ionization era of the universe .