A Model of Metallicity Evolution in the Early Universe

We apply the phenomenological model used to explain the abundances of Fe andr-process elements in very metal-poor stars in the Galaxy to [Fe/H] of dampedLy alpha systems. It is assumed that the first stars formed after the Big Bangwere very massive and promptly enriched the interstellar medium to [Fe/H] ~ -3,at which metallicity formation of normal stars took over. Subsequent Feenrichment was provided by Type II supernovae. The range of [Fe/H] at a givenredshift z for damped Ly alpha systems is explained by the time t* after theBig Bang at which normal star formation started in an individual protogalacticsystem. The average t* is approx 80% the age of the universe for damped Lyalpha systems at z approx 1.5 to 4.5, indicating a long delay between the BigBang and the turn-on of protogalaxies. It is inferred that a substantialfraction of the total baryonic matter may not have been aggregated intoprotogalaxies where normal star formation had occurred down to z ~ 1.5. Thedata near z = 2.2 suggest that the rate of turn-on of protogalaxies wasinitially very low and slowly reached a maximum at ~ 3 Gyr after the Big Bang.This may be important in understanding the rate of formation of quasars.Comment: 13 pages, 4 figures, to appear in ApJ Letter