Direct constraints on high-redshift star formation from observed chemical evolution of DLAs

Damped Lyman-alpha absorbers (DLAs) can be used to measure gas-phase metallicities at large cosmological look-back times with high precision. Such measurements are now available up to z ~ 6. This makes DLAs excellent chemical records of past star formation. In this talk, I will discuss how DLAs can be used to study Population III star formation in the high redshift universe. In particular, I argue that DLAs constrain (1) the initial mass function (IMF) of Population III stars, and (2) their contribution to cosmic reionization. In order to do that, I will present a model of galaxy formation which tracks galactic chemical evolution and the interaction between galaxies and the inter-galactic medium (IGM). This model predicts zero cosmic Population III SFR for z < 8. Nevertheless, at high redshift (z ~ 6) Population III chemical signatures are retained in low-mass galaxies (halo mass ~ 1e9 Msun). We model DLAs as these galaxies and predict distribution of DLA abundance ratios. We find that these distributions are mostly 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 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, and can rule out certain Population III scenarios. DLAs can further be used to constrain the contribution of Population III stars to reionization. Recent studies have highlighted that galaxies at z = 6-8 fall short of producing enough ionizing photons to reionize the IGM, and suggest that Population III stars could resolve this tension. I will argue that, under a variety of assumptions regarding galactic chemical evolution, current chemical data from DLAs rules out fractional contribution of high-mass Population III stars to the ionization rate of more than 10% at z = 10.

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