Hierarchical Structure Formation and Chemical Evolution of Damped Ly alpha Systems

We present a model for chemical evolution of damped Ly alpha systems considering production of metals by SNe II 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 cut-off of infall. The wide range in [Fe/H] at a given redshift is explained by the range of the times for 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 onset of star formation. To reach [Fe/H] = -3 from a metal-free initial state requires only about 30 Myr so that the probability of observing lower [Fe/H] values is very small.Comment: 4 pages, 2 figures, to appear in ApJ

Supernova-driven outflows and chemical evolution of dwarf spheroidal galaxies

We present a general phenomenological model for the metallicity distribution (MD) in terms of [Fe/H] for dwarf spheroidal galaxies (dSphs). These galaxies appear to have stopped accreting gas from the intergalactic medium and are fossilized systems with their stars undergoing slow internal evolution. For a wide variety of infall histories of unprocessed baryonic matter to feed star formation, most of the observed MDs can be well described by our model. The key requirement is that the fraction of the gas mass lost by supernova-driven outflows is close to unity. This model also predicts a relationship between the total stellar mass and the mean metallicity for dSphs in accord with properties of their dark matter halos. The model further predicts as a natural consequence that the abundance ratios [E/Fe] for elements such as O, Mg, and Si decrease for stellar populations at the higher end of the [Fe/H] range in a dSph. We show that for infall rates far below the net rate of gas loss to star formation and outflows, the MD in our model is very sharply peaked at one [Fe/H] value, similar to what is observed in most globular clusters. This suggests that globular clusters may be end members of the same family as dSphs.Comment: 8 pages, 3 figures, to be published in the Proceedings of the National Academy of Science