The Atacama Cosmology Telescope: Reionization kSZ trispectrum methodology and limits

TL;DR

This paper develops methods to detect the reionization kSZ trispectrum in the CMB, addresses contamination challenges, and sets an upper limit using ACT and Planck data, which exceeds simulation predictions.

Contribution

It introduces novel mitigation techniques for foreground and lensing contamination in measuring the reionization kSZ trispectrum in CMB data.

Findings

Upper limit on reionization kSZ trispectrum is 50 times the predicted signal.

Foregrounds and lensing can dominate the trispectrum signal.

Validated mitigation methods on microwave-sky simulations.

Abstract

Patchy reionization generates kinematic Sunyaev-Zeldovich (kSZ) anisotropies in the cosmic microwave background (CMB). Large-scale velocity perturbations along the line of sight modulate the small-scale kSZ power spectrum, leading to a trispectrum (or four-point function) in the CMB that depends on the physics of reionization. We investigate the challenges in detecting this trispectrum and use tools developed for CMB lensing, such as realization-dependent bias subtraction and cross-correlation based estimators, to counter uncertainties in the instrumental noise and assumed CMB power spectrum. We also find that both lensing and extragalactic foregrounds can impart larger trispectrum contributions than the reionization kSZ signal. We present a range of mitigation methods for both of these sources of contamination, validated on microwave-sky simulations. We use ACT DR6 and Planck data to calculate an upper limit on the reionization kSZ trispectrum from a measurement dominated by foregrounds. The upper limit is about 50 times the signal predicted from recent simulations.