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 in the mass range of 100-1000 solar masses, 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 black-body with an effective temperature of ~10⁵ 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 100 solar masses is larger by about an order of magnitude for hydrogen and He I, and by about two 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>6 with the cosmic star formation rate inferred at somewhat lower redshifts.