The Universe Was Reionized Twice

With a top-heavy initial stellar mass function for Population III stars, as recent simulations seem to suggest, we show that Pop III stars can, for the first time, reionize the universe at z~15-20 in the standard cold dark matter cosmological model. The physical reason rests in the positive feedback from Pop III star formation, made possible by a high X-ray background, which, in turn, is produced by supernova blast waves of Pop III stars and miniquasars powered by Pop III black holes both emitting a large fraction of the available energy in X-rays at ~1keV at z~15-25, As a result, H2 molecules can be abundantly produced in minihalos and Pop III star formation activity is able to continue to build up to the required level for reionization. The first reionization by massive Pop III stars heats up the intergalactic medium to a high temperature, sharply raising the Jeans mass and causing star formation rate to plunge. Hence, in the immediate aftermath, ionized hydrogen recombines on a time scale of 10^8yr and the universe once again becomes opaque to Lyman alpha and ionizing photons. The long period from first reionization up to second reionization at z~6, however, does not lack interesting actions. Compton cooling and photoheating by the stars, regulate the Jeans mass and the star formation rate. The mean temperature of the intergalactic medium is maintained at 5,000-10,000K from z~15-20 to z=6. The competition between photoionization and recombination is also at work such that the mean ionization fraction of the intergalactic medium is kept in a rather narrow range ~0.4-1.0 from z=15-20 to z=6. At z~6, the global star formation rate again surpasses the global recombination rate, resulting in the second reionization of the universe. We discuss implications and possible tests for this new reionization picture.

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