CHEMICAL EVOLUTION OF THE HIGH-REDSHIFT GALAXIES

We have tried to determine the rate of chemical evolution of high reds hift galaxies from the observed redshift distribution of the heavy ele ment absorption systems in the spectra of QSOs, taking into account th e evolution in the intensity of the metagalactic UV ionizing radiation background, the radius and/or the co-moving number density of, and th e fraction of mass in the form of gas in, the absorbers. The data for both the Lyman limit systems and the C IV systems have been fitted sim ultaneously. It seems that the abundance of carbon has possibly increa sed by about a factor of 5 to 20 from the cosmic time corresponding to the redshift congruent-to 4 to 2. The data also suggest that either t he radius or the co-moving number density of the galaxies increased wi th redshift up to z = 2.0 and decreased slowly thereafter. The total m ass of the halo gas was higher in the past, almost equal to the entire mass of the galaxy at z = 4. The hydrogen column density distribution for Lyman limit systems predicted by the model is in agreement with t he observed distribution.