Observations of the Lyα forest at z ~ 3 reveal an average metallicity Z ~ 10 -2 Z ☉ . 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:
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