Abstract
In this paper, we explore the constraints on the reionization history that are provided by current observations of the Lyman-alpha forest and the CMB. Rather than using a particular semi-analytic model, we take the novel approach of parametrizing the ionizing sources with arbitrary functions, and perform likelihood analyses to constrain possible reionization histories. We find model independent conclusions that reionization is likely to be mostly complete by z=8 and that the IGM was 50% ionized at z=9-10. Upcoming low-frequency observations of the redshifted 21 cm line of neutral hydrogen are expected to place significantly better constraints on the hydrogen neutral fraction at 6<z<12. We use our constraints on the reionization history to predict the likely amplitude of the 21 cm power spectrum and show that observations with the highest signal-to-noise ratio will most likely be made at frequencies corresponding to z=9-10. This result provides an important guide to the upcoming 21 cm observations. Finally, we assess the impact that measurement of the neutral fraction will have on our knowledge of reionization and the early source population. Our results show that a single measurement of the neutral fraction mid-way through the reionization era will significantly enhance our knowledge of the entire reionization history.
Highlights
Determining how and when the Universe was reionized is one of the major outstanding questions of modern cosmology
We focus on the Lyα forest and cosmic microwave background (CMB) observations which most tightly constrain reionization, and which are most readily predicted by an analytic model
We have developed a framework for combining existing constraints on reionization from CMB and Lyα forest observations, and used them to place bounds on the reionization history
Summary
Determining how and when the Universe was reionized is one of the major outstanding questions of modern cosmology. Having made explicit the bounds of our ignorance in the ionization history, we explore the implications for the signal-to-noise ratio (S/N) that future 21-cm experiments might achieve at different redshifts We turn this problem around and ask what 21-cm experiments will tell us about the reionization history that was not already implicit in our existing observations. This is used to make predictions for upcoming 21-cm instruments.
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