The WMAP satellite has measured a large optical depth to electron scattering after cosmological recombination of \tau _ { es } = 0.17 \pm 0.04 , implying significant reionization of the primordial gas only \sim 200 million years after the big bang .
However , the most recent overlap of intergalactic HII regions must have occurred at z \lesssim 9 based on the Ly \alpha 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 .