STAR FORMATION AT THE TWILIGHT OF THE DARK AGES: WHICH STARS REIONIZED THE UNIVERSE?

摘要

We calculate the global star formation rate density (SFRD from z ～ 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 photoioniz-ing 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 approx< z approx< 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 approx< z approx <20) is approximately equally divided between Population Ⅱ and Population Ⅲ stars, and that if reionization is late (z_(reion) approx< 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.