Monday, March 4, 2013 - 11:00am
"Chemical enrichment of damped Lyman-alpha (DLA) systems as a direct constraint on Population III star formation"
Abstract. Damped Ly-alpha absorbers (DLAs) can be used to measure gas-phase metallicities at large cosmological lookback times with high precision. Relative abundances can still be measured accurately deep into the reionization epoch (z > 6) using transitions redward of Ly-alpha. I will describe the chemical evolution of DLAs using a constrained model for evolution of galaxies and IGM. I will use this model to determine the degree to which DLA abundance measurements can probe Population III enrichment. I will argue that if the critical metallicity of Population III to II transition is < 10^-4 Zsun, the cosmic Population III SFR is zero for z < 8. Nevertheless, at high redshift (z ~ 6) Population III chemical signatures are retained in low-mass galaxies (halo mass < 10^9 Msun). This is because photoionization feedback suppresses star formation in these galaxies until relatively low redshift (z ~ 10), and the chemical record of early Population III star formation is retained. We model DLAs as these galaxies by assigning to them a mass-dependent H I absorption cross section and predict distribution of DLA abundance ratios. These distributions are anchored towards abundance ratios set by Population II yields, but exhibit a tail that depends on the Population III IMF for z > 5. Thus, a sample of DLA abundance measurements at high redshift holds the promise to constrain Population III IMF. A sample of just 10 DLAs with relative abundances measured to an accuracy of 0.1 dex is sufficient to constrain the Population III IMF at 4-sigma. These constraints may prove stronger than other probes such as metal-poor stars and individual DLAs. This approach provides a global picture of the cosmic thermal, ionization, and chemical evolution, and can rule out certain Population III scenarios.