Abstract
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 C IV and Si IV equivalent width values and estimate metallicities of starburst galaxies as a function of redshift. Assuming that a calibration of the C IV 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 at the cosmic epoch corresponding to z 3.2 to about 0.42 Z at z 2.3. A significant further increase in metallicity during later epochs cannot he 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.
Highlights
Among the unsolved questions of cosmology is the beginning and evolution of the star formation process at early cosmic epochs
Assuming that a calibration of the C 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 at the cosmic epoch corresponding to z ≈ 3.2 to about 0.42 Z at z ≈ 2.3
Galaxy counts and emission line studies of high-z galaxies indicate that the star formation rate (SFR) declined rapidly since at least the epoch corresponding to redshifts of z ≈ 2
Summary
Among the unsolved questions of cosmology is the beginning and evolution of the star formation process at early cosmic epochs. Steidel et al (1996a, 1996b) and Lowenthal et al (1997) have demonstrated that galaxies with redshifts up to about z ≈ 5 can be observed with optical photometry and spectroscopy during periods of high star formation activity (resulting in high rest-frame UV emission which is redshifted into the optical wavelength range in the observers frame) Basic properties of these objects, such as number densities, luminosities, colors, sizes, morphologies, star formation rates, overall chemical abundances, dynamics and clustering have been investigated in various recent papers (cf e.g. Steidel et al 1996b; Yee et al 1996; Lowenthal et al 1997; Pettini et al 2000; Leitherer et al 2001).
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