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 Ly alpha 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] similar to -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 Ly alpha 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 approximate to 80% the age of the universe for damped Ly alpha systems at z approximate to 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 similar to 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 similar to 3 Gyr after the big bang. This rate of turn-on of galaxies may be important in understanding the rate of formation of quasars.