Chemical constraints on the contribution of Population III stars to cosmic reionization

Abstract: 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, because their harder spectra can produce ~10x more ionizing photons than Population II. But this argument depends critically on the duration of the Population III era, and because Population III stars form from pristine gas, in turn depends on the rate of galactic enrichment. We use a semi-analytic model of galaxy formation which tracks galactic chemical evolution, to gauge the impact of Population III stars on reionization. Population III SNe produce distinct metal abundances, and we argue that the duration of the Population III era can be constrained by precise relative abundance measurements in high-z damped Ly{\alpha} absorbers (DLAs), which provide a chemical record of past star-formation. We find that a single generation of Population III stars can self-enrich galaxies above the critical metallicity Zcrit=10^-4 Zsun for the Population III-to-II transition, on a very short timescale of ~10^6 yr, owing to the large metal yields and short lifetimes of Population III stars. This subsequently terminates the Population III era, hence they contribute >~ 50% of the ionizing photons only for z >~ 30, and at z=10 contribute <1%. The Population III contribution can be increased by delaying metal mixing into the ISM. However comparing the resulting metal abundance pattern to existing measurements in z <~ 6 DLAs, we show that the fractional contribution of high-mass Population III stars to the ionization rate must be <~ 10% at z = 10. Future abundance measurements of z~7-8 QSOs and GRBs should probe the era when the chemical vestiges of Population III star formation become detectable.