Constraining reionization using 21 cm observations in combination with CMB and Lyman-alpha forest data

TL;DR

This study constrains the reionization history using current Lyman-alpha forest and CMB data, predicts the 21 cm signal, and emphasizes the importance of future measurements for understanding early universe ionization.

Contribution

It introduces a model-independent approach to constrain reionization history using arbitrary functions for ionizing sources and likelihood analysis, providing new insights into reionization timing.

Findings

Reionization is likely complete by z=8.

The IGM was 50% ionized at z=9-10.

Future 21 cm observations will significantly improve constraints.

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.