Reionization by hard photons. I. X-rays from the first star clusters

Observations of the Ly alpha forest at z similar to3 reveal an average metallicity Z similar to 10(-2) Z(circle dot). The high-redshift supernovae that polluted the intergalactic medium also accelerated relativistic electrons. Since the energy density of the cosmic microwave background is proportional to (1+z)(4), at high redshift these electrons cool via inverse Compton scattering. Thus, the first star clusters emit X-rays. Unlike stellar UV ionizing photons, these X-rays can escape easily from their host galaxies. This has a number of important physical consequences: 1. Owing to their large mean free path, these X-rays can quickly establish a universal ionizing background and partially reionize the universe in a gradual, homogeneous fashion. If X-rays formed the dominant ionizing background, the universe would have more closely resembled a single-phase medium rather than a two-phase medium. 2. X-rays can reheat the universe to higher temperatures than possible with UV radiation. 3. X-rays counter the tendency of UV radiation to photodissociate H-2,an important coolant in the early universe, by promoting gas-phase H-2 formation. The X-ray production efficiency is calibrated to local observations of starburst galaxies, which imply that similar to 10% of the supernova energy is converted to X-rays. While direct detection of sources in X-ray emission is difficult, the presence of relativistic electrons at high redshift and thus a minimal level of X-ray emission may be inferred by synchrotron emission observations with the Square Kilometer Array. These sources may constitute a significant fraction of the unresolved hard X-ray background and can account for both the shape and amplitude of the gamma-ray background. This paper discusses the existence and observability of high-redshift X-ray sources, while a companion paper models the detailed reionization physics and chemistry.