Quasars are among the most luminous objects in the universe , and they can be studied in detail up to the highest known redshift .
Assuming that the gas associated with quasars is closely related to the interstellar medium of the host galaxy , quasars can be used as tracer of the star formation history in the early universe .
We have observed a small sample of quasars at redshifts 3 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } z% \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 5 and present results using NV/CIV and NV/HeII as well as MgII/FeII to estimate the date of the first major star formation epoch .
These line ratios indicate solar and supersolar metallicities of the gas close to the quasars .
Assuming times of \tau _ { evol } \simeq 1 Gyr the first star formation epoch can be dated to z _ { f } \simeq 10 , corresponding to an age of the universe of less than 5 \cdot 10 ^ { 8 } yrs ( H _ { o } = 65 km s ^ { -1 } Mpc ^ { -1 } , \Omega _ { M } =0.3 , \Omega _ { \Lambda } = 0.7 ) .