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At redshifts beyond 5–perhaps even beyond 20–stars formed within 'subgalaxies' and created the first heavy elements; these same systems (together perhaps with 'miniquasars') generated the UV radiation that ionized the IGM, and maybe also the first significant magnetic fields. These uncertain processes set the backdrop for the phenomena at z<5 that are the theme of the present conference.
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M. J. Rees (1999) studied this question.