High Redshift Quasars and Star Formation History

Quasars are among the most luminous objects in the uni- verse, 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 ∼ z ∼ 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 ofevol ≃ 1Gyr the first star formation epoch can be dated to zf ≃ 10, corresponding to an age of the universe of less than 5 � 10 8 yrs (Ho = 65kms −1 Mpc −1 , M=0.3, � = 0.7).