Retrieving cosmological information from small-scale CMB foregrounds II. The kinetic Sunyaev Zel'dovich effect

TL;DR

This paper models the kinetic Sunyaev-Zel'dovich effect to extract cosmological information from small-scale CMB data, providing new constraints on reionisation history and the amplitude of the kSZ signal.

Contribution

It introduces a physically motivated, scale-coherent model of reionisation and uses machine learning to infer the kSZ power spectrum from high-resolution CMB data.

Findings

Detection of a 5σ kSZ signal at ℓ=3000 with a value of 3.4 μK²

Upper limit on patchy reionisation contribution: <1.6 μK²

SPT data suggests earlier reionisation than Planck, with τ ≈ 0.062

Abstract

Recent results of ground-based telescopes, giving high-quality measurements of the CMB temperature power spectrum on small scales motivate the need for an accurate model of foregrounds, which dominate the primary signal at these multipoles. In a previous work, we have shown that cosmological information could be retrieved from the power spectrum of the thermal SZ effect. In this work, we introduce a physically motivated model of the Epoch of Reionisation in the cosmological analysis of CMB data, which is coherent on all scales. In particular, at high multipoles, the power spectrum of the kinetic SZ (kSZ) effect is inferred from a set of cosmological and reionisation parameters by a machine-learning algorithm. First including an asymmetric parameterisation of the reionisation history in the Planck 2018 data analysis, we retrieve a value of the optical depth consistent with previous results, but stemming from a completely different history of reionisation in which the first luminous sources light up as early as $z=15$. Considering the latest small-scale data from the SPT and letting the cosmology free to vary, we find that including the new cosmology-dependent SZ spectra helps tighten the constraints on their amplitudes by breaking their degeneracy. We report a $5\sigma$ measurement of the kSZ signal at $\ell=3000$, $\mathcal{D}_{3000}^\mathrm{kSZ} = 3.4^{+0.5}_{-0.3}\,\mu\mathrm{K}^2$ at the 68% confidence level, marginalised over cosmology, as well as an upper limit on the patchy signal from reionisation $\mathcal{D}_{3000}^\mathrm{pkSZ}<1.6~\mu\mathrm{K}^2$ (95% C.L.). Additionally, we find that the SPT data favour slightly earlier reionisation scenarios than Planck, leading to $\tau = 0.062 ^{+0.012}_{-0.015}$ and a reionisation midpoint $z_\mathrm{re} = 7.9^{+1.1}_{-1.3}$ (68% C.L.), which is in line with constraints from high-redshift quasars and galaxies.