Was the universe reionized by massive metal-free stars?

The Wilkinson Microwave Anisotropy Probe satellite has measured a large optical depth to electron scattering after the cosmological recombination of τ_es = 0.17 ± 0.04, implying significant reionization of the primordial gas only ∼200 million years after the big bang. However, the most recent overlap of intergalactic H II regions must have occurred at z ≲ 9 based on the Lyα forest constraint on the thermal history of the intergalactic medium. Here we argue that a first generation of metal-free stars with a heavy (rather than Salpeter) mass function is therefore required to account for much of the inferred optical depth. This conclusion holds if feedback regulates star formation in early dwarf galaxies as observed in present-day dwarfs.