Reionization and the large-scale 21 cm-cosmic microwave background cross correlation

TL;DR

This paper investigates the potential of using the cross-correlation between 21 cm line fluctuations and CMB Doppler signals to study reionization, finding the signal is weak and challenging to detect due to cosmic variance.

Contribution

It introduces a new model for large-scale ionized fraction fluctuations during reionization and analyzes the impact of baryonic velocities on the cross-correlation signal.

Findings

Cross-correlation peaks near the end of reionization.

Signal is strongest at degree scales (~100) and large physical scales (~0.01 Mpc).

Detection is unlikely due to cosmic variance and weak correlation.

Abstract

Of the many probes of reionization, the 21 cm line and the cosmic microwave background (CMB) are among the most effective. We examine how the cross-correlation of the 21 cm brightness and the CMB Doppler fluctuations on large angular scales can be used to study this epoch. We employ a new model of the growth of large scale fluctuations of the ionized fraction as reionization proceeds. We take into account the peculiar velocity field of baryons and show that its effect on the cross correlation can be interpreted as a mixing of Fourier modes. We find that the cross-correlation signal is strongly peaked toward the end of reionization and that the sign of the correlation should be positive because of the inhomogeneity inherent to reionization. The signal peaks at degree scales (l~100) and comes almost entirely from large physical scales (k~0.01 Mpc). Since many of the foregrounds and noise that plague low frequency radio observations will not correlate with CMB measurements, the cross correlation might appear to provide a robust diagnostic of the cosmological origin of the 21 cm radiation around the epoch of reionization. Unfortunately, we show that these signals are actually only weakly correlated and that cosmic variance dominates the error budget of any attempted detection. We conclude that the detection of a cross-correlation peak at degree-size angular scales is unlikely even with ideal experiments.