A Model of Metallicity Evolution in the Early Universe

We apply the phenomenological model used to explain the abundances of Fe and r-process elements in very metal-poor stars in the Galaxy to [Fe/H] of damped Lya systems. It is assumed that the first stars that formed after the big bang were very massive and that they promptly enriched the interstellar medium to , at which [Fe/H] ∼ 23 metallicity the formation of normal stars took over. Subsequent Fe enrichment was provided by Type II supernovae. The range of [Fe/H] at a given redshift z for damped Lya systems is explained by the time after the big bang ? t at which normal star formation started in an individual protogalactic system. The average is ≈80% the age of ? t the universe for damped Lya systems at –4.5, indicating a long delay between the big bang and the turnz ≈ 1.5 on of protogalaxies. It is inferred that a substantial fraction of the total baryonic matter may not have been aggregated into protogalaxies with normal star formation events until a late time corresponding to . The data near z ∼ 1.5 indicate that the rate of turn-on of protogalaxies was initially very low and that it slowly reached a maximum z p 2.2 at ∼3 Gyr after the big bang. This rate of turn-on of galaxies may be important in understanding the rate of formation of quasars. Subject headings: galaxies: abundances — galaxies: evolution — quasars: absorption lines