A Model of Metallicity Evolution in the Early Universe

Abstract

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 Lyα 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 [Fe/H] ~ -3, at which 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 Lyα systems is explained by the time t* after the big bang at which normal star formation started in an individual protogalactic system. The average t* is ≈80% the age of the universe for damped Lyα systems at z ≈ 1.5-4.5, indicating a long delay between the big bang and the turn-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 z ~ 1.5. The data near z = 2.2 indicate that the rate of turn-on of protogalaxies was initially very low and that it slowly reached a maximum 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.

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