The history of the transition from a neutral intergalactic medium (IGM) to one that is almost fully ionized can reveal the character of cosmological ionizing sources and set important constraints on the stellar birthrate at high redshifts. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of photons emitted above 1 ryd in one recombination time equals the mean number of hydrogen atoms. If stellar sources are responsible for photoionizing the IGM at z = 5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z ≈ 3, and the mean metallicity per baryon in the universe ∼> 1/500 solar. In hierarchical clustering scenarios, high-z dwarfs (i.e. an early generation of stars in dark matter halos with circular velocities vcirc ≈ 50 km s −1) are expected to be one of the main source of UV photons and heavy elements at early epochs. They would be very numerous, ∼> 0.2 arcsec −2, and faint, IAB ∼> 29.5 mag: their detection may have to wait for the Next Generation Space Telescope.
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