Probing Reionization with Currently Available Observables

Abstract

The reionization of hydrogen -- the era when the intergalactic gas turned from cold and neutral to hot and highly ionized when the universe was hundreds of millions of years old -- remains one of the biggest mysteries in astronomy research. Until observations of the 21 cm transition of neutral hydrogen become available, we do not have a probe that can directly image the reionization process. In this thesis, I discuss what imprints reionization can leave on currently available observables, and how we might use these signatures to constrain reionization. The observables I study include the high-redshift galaxy UV luminosity function, the Lyman-alpha forest, and the large-angular scale E-mode polarization of the cosmic microwave background. These observables probe different aspects of reionization. The galaxy UV luminosity function may contain imprints of photoheating feedback during reionization, which is expected to suppress the faint-end slope of the luminosity function. The Lyman-alpha forest can be used to study the patchiness of reionization. The large-angular cosmic microwave background E-mode polarization might have information about Population III reionization at z>15. To study these, I have used a combination of numerical simulations and semi-analytic calculations. I also comment on the accuracy of moment-based radiative transfer algorithms on simulating reionization, using analytic calculations and toy models designed to mimic the post-reionization gas.