Reionization on Large Scales II: Detecting Patchy Reionization through Cross Correlation of the Cosmic Microwave Background

TL;DR

This paper proposes a method to detect patchy reionization effects on the CMB by analyzing cross correlations between large and small scales, accounting for secondary anisotropies, and demonstrating potential detectability with current and future observations.

Contribution

It introduces a novel patchy tau correlator technique that enhances detection sensitivity of reionization patchiness in the CMB, considering secondary anisotropies and different reionization models.

Findings

Patchy reionization transfers power from large to small scales in the CMB.

The patchy tau correlator is sensitive to small RMS optical depth values (~0.003).

Models with different tau_rms values are distinguishable even with the same mean tau.

Abstract

We investigate the effect of patchy reionization on the cosmic microwave background temperature. An anisotropic optical depth tau (theta) alters the TT power spectrum on small scales l > 2000. We make use of the correlation between the matter density and the reionization redshift fields to construct full sky maps of tau(theta). Patchy reionization transfers CMB power from large scales to small scales, resulting in a non-zero cross correlation between large and small angular scales. We show that the patchy tau correlator is sensitive to small root mean square values tau_rms ~ 0.003 seen in our maps. We include other secondary anisotropies such as CMB lensing, kinetic and thermal Sunyaev-Zel'dovich terms, as well as the infrared and point source background, and show that patchy reionization may be detected in the low frequency channels ~ 90 GHz, particularly for extended reionization histories. If frequency dependent secondaries can be minimized by a multi-frequency analysis, we show that even small degrees of patchiness can be detected at high significance. In this case, we show that models with different values of tau_rms are distinguishable, even for the same mean value <tau>. The patchy tau correlator thus provides information that is complementary to what may be obtained from the polarization and the kinetic Sunyaev-Zel'dovich power spectra.