Star formation at the twilight of the dark ages: Which stars reionized the universe?

We calculate the global star formation rate density (SFRD) from z similar to 30-3 using a semianalytic model incorporating the hierarchical assembly of dark matter halos, gas cooling via atomic hydrogen, star formation, supernova feedback, and suppression of gas collapse in small halos due to the presence of a photoionizing background. We compare the results with the predictions of simpler models based on the rate of dark matter halo growth and a fixed ratio of stellar to dark mass, and with observational constraints on the SFRD at 3 less than or similar to z less than or similar to 6. We also estimate the star formation rate due to very massive, metal-free Population III stars using a simple model based on the halo formation rate, calibrated against detailed hydrodynamic simulations of Population III star formation. We find that the total production rate of hydrogen-ionizing photons during the probable epoch of reionization (15 less than or similar to z less than or similar to 20) is approximately equally divided between Population II and Population III stars, and that if reionization is late (z(reion) less than or similar to 15, close to the lower limit of the range allowed by the Wilkinson Microwave Anisotropy Probe results), then Population II stars alone may be able to reionize the universe.