THE CHEMICAL EVOLUTION OF VERY METAL-POOR DAMPED LYα SYSTEMS

In earlier work we showed that a dark matter halo with a virial mass of 107 M. can survive feedback from its own massive stars and form stars for. 100 Myr. We also found that our modeled systems were consistent with observations of ultrafaint dwarfs (UFDs), the least massive known galaxies. Very metal-poor damped Lya systems (DLAs) recently identified at z similar to 2 may represent the gas that formed at least some of the observed stars in UFDs. We compare projected sightlines from our simulations to the observed metal-poor DLAs and find that our models can reach the densities of the observed sightlines; however the metallicities are inconsistent with the single supernova simulations, suggesting enrichment by multiple supernovae. We model two scenarios for the history of these systems. The first explains the gas abundances in DLAs by a single burst of star formation. This model can produce the observed DLA abundances, but does not provide an explanation as to why the DLAs show suppressed [alpha/Fe] compared to the stellar population of UFDs. The second scenario splits the DLAs into a population which is enriched by a single burst, and a population that is enriched by a second burst after the accretion of metal-poor gas. In this scenario, the suppressed average [alpha/Fe] in DLAs compared to UFDs results from enrichment of second-burst systems by Type Ia supernovae.