Bias to CMB lensing reconstruction from temperature anisotropies due to large-scale galaxy motions

Abstract

Gravitational lensing of the cosmic microwave background (CMB) is expected to be amongst the most powerful cosmological tools for ongoing and upcoming CMB experiments. In this work, we investigate a bias to CMB lensing reconstruction from temperature anisotropies due to the kinematic Sunyaev-Zel'dovich (kSZ) effect, that is, the Doppler shift of CMB photons induced by Compton-scattering off moving electrons. The kSZ signal yields biases due to both its own intrinsic non-Gaussianity and its non-zero cross-correlation with the CMB lensing field (and other fields that trace the large-scale structure). This kSZ-induced bias affects both the CMB lensing auto-power spectrum and its cross-correlation with low-redshift tracers. Furthermore, it cannot be removed by multifrequency foreground separation techniques because the kSZ effect preserves the blackbody spectrum of the CMB. While statistically negligible for current datasets, we show that it will be important for upcoming surveys, and failure to account for it can lead to large biases in constraints on neutrino masses or the properties of dark energy. For a Stage 4 CMB experiment, the bias can be as large as ≈ 15% or 12% in cross-correlation with LSST galaxy lensing convergence or galaxy overdensity maps, respectively, when the maximum temperature multipole used in the reconstruction is max = 4000, and about half of that when max = 3000. Similarly, we find that the CMB lensing auto-power spectrum can be biased by up to several percent. These biases are many times larger than the expected statistical errors. We validate our analytical predictions with cosmological simulations and present the first complete estimate of secondary-induced CMB lensing biases. The predicted bias is sensitive to the small-scale gas distribution, which is affected by pressure and feedback mechanisms, thus making removal via "bias-hardened" estimators challenging. Reducing max can significantly mitigate the bias at the cost of a decrease in the overall lensing reconstruction signal-tonoise. A bias 1% on large scales requires max 2000, which leads to a reduction in signal-to-noise by a factor of ≈ 3 − 5 for a Stage 4 CMB experiment. Polarization-only reconstruction may be the most robust mitigation strategy.

Baryonic effects on CMB lensing and neutrino mass constraints

“…To make good on this promise, however, upcoming lensing measurements will need to control for a plethora of systematics related to beam calibration [14], galactic and extragalactic foregrounds [15][16][17], and non-Gaussian clustering of the lenses [18,19]. In addition, we must have confidence in our modelling of the lensing signal itself.…”

Dark Energy Survey year 1 results: Joint analysis of galaxy clustering, galaxy lensing, and CMB lensing two-point functions

“…Finally, we note again that the dominant source of bias in the measurements of the CMB lensing cross-correlations presented here is contamination of the κ CMB map from O17 by the tSZ effect. Several recent works have considered related biases in measurements of the CMB lensing autospectrum and have developed potential mitigation schemes [59][60][61]. Other recent work has considered biases in CMB lensing cross-correlations with large scale structure observables and possible mitigation strategies such as multi-frequency foreground cleaning [62] and shear-based estimation [63].…”

Modeling CMB lensing cross correlations with CLEFT

“…Throughout this paper we shall follow standard practice and approximate the lensing using the Born approximation, though we shall include non-linear terms in the large-scale densities. As the precision improves it will be necessary to reconsider all such approximations [59,60] for cross-correlations as well as the auto-spectrum of κ and to worry about cleaning out contaminants [5,61,62]. Isolating the signal to higher redshift, where the non-linearity is less pronounced, makes the cross-spectrum less sensitive to bispectrum and trispectrum terms than the κ auto-spectrum.…”