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We calculate the generic spectral signature of an early population of massivestars at high redshifts. For metal-free stars in the mass range of 100-1000solar masses, we find that the combined spectral luminosity per unit stellarmass is almost independent of the mass distribution of these stars. To zerothorder, the generic spectrum resembles a black-body with an effectivetemperature of ~10^5 K, making these stars highly efficient at ionizinghydrogen and helium. The production rate of ionizing radiation per stellar massby stars more massive than 100 solar masses is larger by about an order ofmagnitude for hydrogen and He I, and by about two orders of magnitude for HeII, than the emission from a standard initial mass function. This would resultin unusually strong hydrogen and helium recombination lines from thesurrounding interstellar medium. It could also alleviate the current difficultyof ionizing the intergalactic medium at z>6 with the cosmic star formation rateinferred at somewhat lower redshifts.Comment: ApJ in press, 11 emulated ApJ pages, 4 figures, minor revision

Generic Spectrum and Ionization Efficiency of a Heavy Initial Mass Function for the First Stars