Generic spectrum and ionization efficiency of a heavy initial mass function for the first stars

We calculate the generic spectral signature of an early population of massive stars at high redshifts. For metal-free stars with mass above 300 Mo. we find that the combined spectral luminosity per unit stellar mass is almost independent of the mass distribution of these stars. To zeroth order, the generic spectrum resembles a blackbody with an effective temperature of similar to 10(5) K, making these stars highly efficient at ionizing hydrogen and helium. The production rate of ionizing radiation per stellar mass by stars more massive than similar to 300 M. is larger by similar to1 order of magnitude for hydrogen and He I and by similar to2 orders of magnitude for He II than the emission from a standard initial mass function. This would result in unusually strong hydrogen and helium recombination lines from the surrounding interstellar medium. It could also alleviate the current difficulty of ionizing the intergalactic medium at z greater than or similar to 6 with the cosmic star formation rate inferred at somewhat lower redshifts.