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 0.16 Z_sun at the cosmic epoch corresponding to z ~ 3.2 to about 0.42 Z_sun 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.