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Reionization of the Intergalactic Medium

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Hannah KrugASTR 688RSpring 2008Final ProjectDue 5/13/08Reionization of the Intergalactic Medium:What Is it and When Did it Occur?In the time following the Big Bang, there are two epochs which astronomers can consider arguably the most important cosmologically: the epoch of recombination and the epoch of reionization. But while recombination, the time when free protons and electrons coupled to form neutral hydrogen throughout the universe, has been well studied and pinpointed by WMAP (as we have discussed thoroughly in class), determining when reionization, the period in which the universe switched from neutral hydrogen back to an ionized plasma, occurred has proven much more elusive. Much work is currently being done to try to solve this mystery, including here at Maryland; the NEWFIRM instrument was developed in part for this purpose and I plan to make use of it for my PhD thesis. In this paper, I will outline the importance of the reionization epoch and the basic work being done to attempt to determine when this important epoch occurred.In current cold dark matter cosmology, the universe is believed to have formed in a “bottom-up” hierarchy: the smallest objects formed first and large scale structures such as galaxy clusters and filaments formed at later times, with all structures stemming originally from density perturbations. Once the original stars formed, it is believed that those objects (or others, as will be described) provided the energy which ionized neutral hydrogen and allowed the universe to become transparent to photons once again, thus ending the cosmological “dark ages” (Loeb & Barkana 2001 reionization review, hereafter LB). The starting point to probing this period of reionization comes from the fact that the ionization energy of hydrogen is 13.6 electron volts and that the nuclear fusion reactions that take place in the center of stars release on the order of one hundred thousand times that energy amount; the primary assumption is that stars alone could have been responsible for this reionization epoch. Had reionization never occurred and space stayed opaque to photons, the universe would be a very different place and large scale structure may never have been able to develop, and thus pinpointing when this epoch took place is of great interest to astronomers. In 2001, astronomers believed reionization to have taken place sometime between redshifts 30 and 7, a wide range, one which more recent efforts have attempted to narrow (LB). One result that has helped to narrow down the predicted reionization redshift range comes from the important Wilkinson Microwave Anisotropy Probe, or WMAP. Not only did WMAP constrain important cosmological parameters (the omega values which we have used in class and which are used in all the studies which will be described here), but from its measurements of the cosmic microwave background, or CMB, a neutral universe is inferred for redshifts approximately greater than or equal to 14 (Cantalupo, Porciani, & Lilly 2007). A rough lower limit can be determinedby applying the Gunn-Peterson test to quasars. With the Lyman- atomic transition, even a region with aα small neutral hydrogen number density can show a strong resonant scattering feature which simulates absorption at the transition line. One would normally expect a sharp extinction drop between the long-wavelength and short-wavelength sides of the Lyman- emission line, but due to this resonant scatteringα feature, we do not observe that. The Gunn-Peterson test can thus be used to provide an upper limit for the neutral hydrogen density (Coles & Lucchin 2002). Applying the Gunn-Peterson test to quasars at around redshift 6 has shown a very low neutral hydrogen fraction at that time, implying that the universe has already been ionized (Cantalupo et al. 2007). Knowledge of how exactly reionization occurred can help cosmologists more accurately pinpoint when it occurred. In standard reionization theory, an “intergalactic ionizing radiation field” is assumed to have spread throughout the universe, ionizing hydrogen as it propagated. Astronomers are thus faced with a major problem from the start, because this ionizing field is made up of ionizing radiation which has escaped from stars and quasars, but determining the fraction of ionizing radiation that escapes from these stars and quasars is very difficult and has not yet been well constrained (LB). Attempts to constrain this escape fraction have included looking at theoretical models of the Milky Way's own O and B stars (since the hottest stars would have been those which produced the ionizing radiation), giving 3-14% escape fraction, measuring H- lines in the Magellanic Stream, giving 6% escape fraction, looking atα nearby starburst galaxies, giving a range between 3 and 57 percent, and making the assumption that galaxies are isotropic point sources, which yields a 5-60% escape fraction (LB). This extremely wide range, 3-60%, is a problem in and of itself, but becomes even worse due to the fact that the distribution of gas in these galaxies is thought to play a major role in determining the escape fraction. High redshift stars and galaxies that would be producing the ionization fronts may have very different gas distributions than galaxies in our local universe, so the escape fraction could be much lower (LB). The escape fraction used in reionization models plays a major role in the calculation of the reionization redshift, so better constraints on that fraction are crucial to a theoretical understanding of the reionization epoch. More recent studies have limited this escape fraction to under 20%, based on Lyman- opacity evolution andα the amount of ultraviolet radiation found in the cosmic background (Ciardi, Ferrara, & White 2003).The reionization epoch is not believed to have occurred all at once, but rather believed to have progressed through a series of stages. In the beginning, individual sources would begin producing ionization radiation during the “pre-overlap” phase, and it is thought that these first sources would come from regions with the highest density and most massive dark matter halos, according to Gnedin as cited in Loeb & Barkana. The ionization fronts move slowly at first, as they are originating in high-density environments, and then progress faster once they reach regions of low density. The IGM is considered to be a “two-phase medium” at this point


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