Enough OB stars must have formed before z = 5 to produced the UV radiation that ionized the intergalactic medium. The initial mass function for these Population III stars is uncertain: if it rose steeply towards low masses (or if much of the early UV was reabsorbed in dense clouds), a substantial fraction of the primordial baryons could have been converted into brown dwarfs (MACHOs) during the pregalactic era.
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