Evidence for chemical evolution in the spectra of high redshift galaxies

Using a sample of 57 VLT FORS spectra in the redshift range 1:37< z< 3:40 (selected mainly from the FORS Deep Field survey) and a comparison sample with 36 IUE spectra of local (z 0) starburst galaxies we derive Civ and Siiv equivalent width values and estimate metallicities of starburst galaxies as a function of redshift. Assuming that a calibration of the Civ equivalent widths in terms of the metallicity based on the local sample of starburst galaxies is applicable to high-z objects, we find a significant increase of the average metallicities from about 0:16Z at the cosmic epoch corresponding to z 3:2 to about 0:42Z at z 2:3. A significant further increase in metallicity during later epochs cannot be detected in our data. Compared to the local starburst galaxies our high-redshift objects tend to be overluminous for a fixed metallicity. Our observational results are in good agreement with published observational data by other authors and with theoretical predictions of the cosmic chemical evolution.