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Reionization - Reionization The lack of a Gunn-Peterson...

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Reionization The lack of a Gunn-Peterson trough in the spectra of quasars long ago established that the intergalactic medium must be highly ionized. Since we see radiation from the initial recombination as the cosmic microwave background, it must have been reionized sometime later. This reionization epoch is important for the history of galaxy assembly and our ability to observe high-redshift objects; its progress likewise carries clues to the early history of galaxies and QSOs. Numerical modelling of the recombination era tracks the same features seen in modelling H II regions - expanding ionization fronts, surrounding partially ionized zones - now set against the backdrop of cosmic expansion. The results have been reviewed in detail by Loeb & Barkana (2001 ARA&A 39, 19). As the ionized volumes ("Strömgren spheres") expand around individual ionizing sources, it is useful to follow Gnedin (2000 ApJ 535, 530) in distinguishing phases before, during, and after the overlap of these volumes. Initially, each ionized bubble expands independently, fully surrounded by neutral material. The rate of expansion depends on the ionizing luminosity, shape of the ionizing spectrum, and density distribution of the neutral gas.
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