The Universe Was Reionized Twice

We show that the universe was reionized twice, first at z ~ 15-16 and again at z ~ 6. Such an outcome appears inevitable when normalizing to two well-determined observational measurements, namely, the epoch of the final cosmological reionization at z ~ 6 and the density fluctuations at z ~ 6, which in turn are tightly constrained by Lyα forest observations at z ~ 3. These two observations most importantly fix the product of star formation efficiency and the ionizing photon escape fraction from galaxies at high redshift. The only major assumption made is that the initial mass function of metal-free, Population III stars is top-heavy. To the extent that the relative star formation efficiencies in gaseous minihalos with H2 cooling and large halos with atomic cooling at high redshift are unknown, the primary source for the first reionization is still uncertain. If star formation efficiency in minihalos is at least 10% of that in large halos, then Population III stars in the minihalos may be largely responsible for the first reionization; otherwise, the first reionization will be attributable largely to Population III stars in large halos. In the former case, H2 cooling in minihalos is necessarily efficient. We show that gas in minihalos can be cooled efficiently by H2 molecules and that star formation can continue to take place largely unimpeded throughout the first reionization period, as long as gas is able to accumulate in them. This comes about thanks to two new mechanisms for generating a high X-ray background during the Population III era put forth here, namely, X-ray emission from the cooling energy of Population III supernova blast waves and that from miniquasars powered by Population III black holes. Consequently, H2 formation in the cores of minihalos is significantly induced to be able to counteract the destruction by Lyman-Werner photons produced by the same Population III stars. In addition, an important process for producing a large number of H2 molecules in relic H II regions of high-redshift galaxies, first pointed out by Ricotti, Gnedin, & Shull in 2001, is quantified here for Population III galaxies. It is shown that H2 molecules produced by this process may overwhelm the dissociating effects of the Lyman-Werner photons produced by stars in the same Population III galaxies. As a result, the Lyman-Werner background may not build up in the first place during the Population III era. The long cosmological reionization and reheating history is complex. From z ~ 30, Population III stars gradually heat up and ionize the intergalactic medium, completing the first reionization at z ~ 15-16, followed by a brief period of Δz ~ 1, during which the intergalactic medium stays completely ionized because of sustained ionizing photon emission from concomitant Population III galaxies. The transition from Population III stars to Population II stars at z ~ 13 suddenly reduces, by a factor of ~10, the ionizing photon emission rate, causing hydrogen to rapidly recombine, marking the second cosmological recombination. From z ~ 13 to 6, Compton cooling by the cosmic microwave background and photoheating by the stars self-regulate the Jeans mass and the star formation rate, giving rise to a mean temperature of the intergalactic medium maintained nearly at a constant of ~104 K. Meanwhile, recombination and photoionization balance one another such that the intergalactic medium stays largely ionized during this stage, with n/nH ≥ 0.6. Most of the star formation in this period occurs in large halos with dominant atomic line cooling. We discuss a wide range of implications and possible tests for this new reionization picture. In particular, the Thomson scattering optical depth is increased to 0.10 ± 0.03, compared to 0.027 for the case of only one rapid reionization at z = 6. Upcoming Wilkinson Microwave Anisotropy Probe observations of the polarization of the cosmic microwave background should be able to distinguish between these two scenarios. In addition, properties of minihalos at high redshift (z ≥ 6) will be very different from previous expectations; in particular, they will be largely deprived of gas, perhaps alleviating the cosmological overcooling problem.