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Observations of the Ly-alpha forest at z~3 reveal an average metallicityZ~0.01 Z_solar. The high-redshift supernovae that polluted the IGM alsoaccelerated relativistic electrons. Since the energy density of the CMB scalesas (1+z)^4, at high redshift these electrons cool via inverse Comptonscattering. Thus, the first star clusters emit X-rays. Unlike stellar UVionizing photons, these X-rays can escape easily from their host galaxies. Thishas a number of important physical consequences: (i) Due to their large meanfree path, these X-rays can quickly establish a universal ionizing backgroundand partially reionize the universe in a gradual, homogeneous fashion. IfX-rays formed the dominant ionizing background, the universe would have moreclosely resembled a single-phase medium, rather than a two-phase medium. (ii)X-rays can reheat the universe to higher temperatures than possible with UVradiation. (iii) X-rays counter the tendency of UV radiation tophoto-dissociate H2, an important coolant in the early universe, by promotinggas phase H2 formation. The X-ray production efficiency is calibrated to localobservations of starburst galaxies, which imply that ~10% of the supernovaenergy is converted to X-rays. While direct detection of sources in X-rayemission is difficult, the presence of relativistic electrons at high redshiftand thus a minimal level of X-ray emission may be inferred by synchrotronemission observations with the Square Kilometer Array. These sources mayconstitute a significant fraction of the unresolved hard X-ray background, andcan account for both the shape and amplitude of the gamma-ray background. Thispaper discusses the existence and observability of high-redshift X-ray sources,while a companion paper models the detailed reionization physics and chemistry.Comment: Final version accepted by ApJ. 32 pages, 3 figure

Reionization by Hard Photons. I. X‐Rays from the First Star Clusters