Hierarchical Structure Formation and Chemical Evolution of Damped Lyα Systems

We present a model for the chemical evolution of damped Lyα systems considering the production of "metals" by Type II supernovae and infall associated with hierarchical structure formation. The growth of metallicity in these systems is a reflection of the competition between astration and infall. The apparent late turn-on of these systems is due to the late cutoff of infall. The wide range in [Fe/H] at a given redshift is explained by the range of the times for the onset of star formation and the range of the times for infall cessation in different systems. The observed lower bound of [Fe/H] ≈ -3 follows from the very rapid initial rise of [Fe/H] subsequent to the onset of star formation. To reach [Fe/H] ≈ -3 from a metal-free initial state requires only ~30 Myr, so the probability of observing lower [Fe/H] values is very small.