The Universe Was Reionized Twice

By Renyue Cen. 2002

240 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/5bae90e1f5136a853cad6a3300eb91cab8687971

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Citations in the corpus

Fossil H ii regions: self-limiting star formation at high redshift (2003, methodology): “This is similar to the reionization histories derived by (Cen 2003; Wyithe & Loeb 2003), but is regulated by feedback from the entropy floor rather than a Pop III to Pop II transition due to a universal metallicity increase.”
The Evolution of the Intergalactic Medium (2015, background): “This tension led to a plethora of exotic solutions (Cen 2003; Haiman & Holder 2003).”
Implications for Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3 (2005, background): “Som etheoristshaveargued thatthe hydrogen in the IG M could have been reionized twice(W yithe & Loeb 2003;Cen 2003).”
Comparison of reionization models: radiative transfer simulations and approximate, seminumeric models (2010, background): “Taken at face value, these observations favor an extended reionization epoch (see also Cen 2003; Fan et al. 2006; Bolton & Haehnelt 2007b; Wyithe & Cen 2007).”
Self-regulated reionization (2006, background): “However, none of our models show the double reionization predicted by Wyithe & Loeb (2003) and Cen (2003).”
The First Cosmic Structures and Their Effects (2004, background): “Madau & Rees (2000) and Cen & Haiman (2000) (see also Cen 2003c) show that a sufficiently bright quasar placed at z > 6 can produce a Strömgren sphere large enough to render its Lyα emission line detectable, e.g. by JWST or Keck.”
LOWER-LUMINOSITY GALAXIES COULD REIONIZE THE UNIVERSE: VERY STEEP FAINT-END SLOPES TO THE UV LUMINOSITY FUNCTIONS AT z ⩾ 5–8 FROM THE HUDF09 WFC3/IR OBSERVATIONS (2011, background): “These include an even smaller clumping factor (e.g., Bolton & Haehnelt 2007; Pawlik et al. 2009) and perhaps a contribution from early population III stars to the UV photon flux (e.g., Cen 2003; Ricotti & Ostriker 2004).”
The First Stars (2003, background): “It has been suggested that an early generation of very massive Population III stars, as predicted by numerical simulations (see Section 3), could naturally allow for a solution (Cen 2003a,b; Haiman & Holder 2003; Somerville & Livio 2003; Wyithe & Loeb 2003a,b).”
Early reionization by the first galaxies (2003, background): “A fascinating (although speculative) possibility is that the Universe was reionized twice (Cen 2003; Wyithe & Loeb 2003) with a relatively short redshift interval in which the IGM became neutral again.”
IONIZING PHOTON ESCAPE FRACTIONS FROM HIGH-REDSHIFT DWARF GALAXIES (2008, background): “…it has to be assumed that the product of star formation efficiency and hydrogen ionizing photon escape fraction is ≥ 0.01 (e.g. Gnedin 2000; Cen 2003a,b; Wyithe & Loeb 2003a,b; Venkatesan et al. 2003; Somerville et al. 2003; Chiu et al. 2003; Haiman & Holder 2003; Ciardi et al. 2003;…”
CRITICAL STAR FORMATION RATES FOR REIONIZATION: FULL REIONIZATION OCCURS AT REDSHIFT z ≈ 7 (2012, background): “…of high-z galaxies leave open several ionization scenarios, some involving simple hydrogen reionization at z ≈ 10 and He ii reionization at z ≈ 3, and others with more complex ionization histories (Bolton & Haehnelt 2007; Venkatesan et al. 2003; Cen 2003) that depend on SFRs at z = 7–20.”
Taxing the rich: Recombinations and bubble growth during reionization (2005, background): “The most popular solution is to invoke some sort of feedback mechanism to decrease the ionizing efficiency of galaxies during reionization (e.g. Wyithe & Loeb 2003a,b; Cen 2003a,b; Haiman & Holder 2003).”
Constraining the Evolution of the Ionizing Background and the Epoch of Reionization with z ∼ 6 Quasars. II. A Sample of 19 Quasars (2005, methodology): “The WMAP polarization results, combined with low redshift constraints, suggest that reionization could be an extended process rather than a phase transition (Cen 2003a, b, Wyithe & Loeb 2003a, Haiman & Holder 2003, Somerville et al. 2003, Gnedin 2004, Choudhury & Ferrara 2005).”
Reionization and Galaxy Evolution Probed by z = 7 Lyα Emitters (2007, background): “…with WMAP only if there is a statistical fluke (one time in 20, a 95% confidence range is wrong) or the reionization happened twice (e.g., Cen 2003) so that a lot of the observed electron scattering happens at z ≫ 7, and then the universe becomes partially neutral again, allowing us to…”
STATISTICS OF 207 Lyα EMITTERS AT A REDSHIFT NEAR 7: CONSTRAINTS ON REIONIZATION AND GALAXY FORMATION MODELS (2010, methodology): “Cen (2003) argues that reionization took place twice; the first at z ∼ 15 − 16 by metal-free Population III stars with a top-heavy IMF and the second at z ∼ 6 by Population II stars, which give a medium large Thomson scattering optical depth of 0.10 ± 0.03 similar to WMAP7 results (Larson et al.…”
Observational Constraints on Cosmic Reionization (2006, background): “…C., Gnedin, N., Furlanetto, S., & Owen, F. 2004, NewAR, 48, 1053-1061 Carilli et al. 2004bCarilli, C., Furlanetto, S., Briggs, F., Jarvis, M., Rawlings, S., Falcke, H. 2004, NewAR, 48, 1029-1038 Carilli et al. 2002Carilli, C., Gnedin, N., Owen, F. 2002, ApJ, 577, 22-30 Cen 2003aCen, R. 2003a, Ap.”
The global 21-centimeter background from high redshifts (2006, background): “Early efforts suggested that they might introduce sharp features such as ‘double reionization’ (Cen 2003b; Wyithe & Loeb 2003a).”