  We present results of the analysis of near infrared spectroscopic
observations of 6 high-redshift quasars (z > 4), emphasizing the measurement of
the ultraviolet FeII/MgII emission line strength in order to estimate the
beginning of intense star formation in the early universe. To investigate the
evolution of the FeII/MgII ratio over a wider range in cosmic time, we measured
this ratio for composite quasar spectra which cover a redshift range of 0 < z <
5 with nearly constant luminosity, as well as for those which 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 FeII/MgII 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 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 0.2 to 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 to 9, when the age of the universe
was ~0.5 Gyr (H_o = 72 km/s/Mpc, Omega_M = 0.3, Omega_Lambda = 0.7). This epoch
corresponds well to the re-ionization era of the universe.

Astrophysics