First Star Clusters

Observations of the Lyα forest at z ∼ 3 reveal a metallicity Z ∼ 10 −2 Z ⊙. The high-redshift supernovae that polluted the IGM also accelerated relativistic electrons. Since the energy density of the CMB ∝ (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. Due to their large mean free path, these X-rays would have reionized the universe in a gradual, homogeneous fashion, regardless of the spatial distribution of sources. In this scenario, the ionization fraction of the single-phase IGM rises gradually, in contrast to canonical models where HII regions expand in a neutral IGM. X-rays also played an important role in reheating the universe to higher temperatures than possible with UV radiation. In particular, they may be responsible for observed Lyα forest line widths in excess of predictions from hydrodynamic simulations. These X-rays also counter the tendency of UV radiation to photo-dissociate H 2 , an important coolant in the early universe, by promoting gas phase H 2 formation. 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 (Oh 2000, in preparation) models the detailed reionization physics and chemistry.