Pritchard, Jonathan R.Loeb, AbrahamWyithe, J. Stuart B.2026-01-022026-01-020035-8711ORCID:/0000-0001-7956-9758/work/195569852https://hdl.handle.net/1885/733802390In this paper, we explore the constraints on the reionization history that are provided by current observations of the Lyα forest and the cosmic microwave background. 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 intergalactic medium was 50 per cent 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.14enCosmology: theoryDiffuse radiationGalaxies: high-redshiftIntergalactic medium201010.1111/j.1365-2966.2010.17150.x78649427054