Impact of thermal SZ effect on cross-correlations between Planck CMB lensing and SDSS galaxy density fields

TL;DR

Residual thermal Sunyaev-Zeldovich contamination in CMB maps biases the cross-correlation measurements with large-scale structure, affecting cosmological inferences, and requires effective deprojection techniques for accurate analysis.

Contribution

This study quantifies the impact of residual tSZ contamination on CMB lensing and galaxy cross-correlations, highlighting the necessity of tSZ deprojection for unbiased results.

Findings

Residual tSZ causes a ~2.5% negative bias at low multipoles.

A ~9% positive bias occurs at high multipoles due to tSZ contamination.

Masking clusters does not fully eliminate the tSZ-induced bias.

Abstract

Residual foreground contamination by thermal Sunyaev-Zeldovich (tSZ) effect from galaxy clusters in cosmic microwave background (CMB) maps propagates into the reconstructed CMB lensing field, and thus biases the intrinsic cross-correlation between CMB lensing and large-scale structure (LSS). Through stacking analysis, we show that residual tSZ contamination causes an increment of lensing convergence in the central part of the clusters and a decrement of lensing convergence in the cluster outskirts. We quantify the impact of residual tSZ contamination on cross-correlations between the Planck 2018 CMB lensing convergence maps and the SDSS-IV galaxy density data through cross-power spectrum computation. In contrast with the Planck 2018 tSZ-deprojected SMICA lensing map, our analysis using the tSZ-contaminated SMICA lensing map measures a $\sim2.5\%$ negative bias at multipoles $\ell\lesssim 500$ and transits to a $\sim9\%$ positive bias at $\ell\gtrsim1500$, which validates earlier theoretical predictions of the overall shape of such tSZ-induced spurious cross-correlation. The tSZ-induced lensing convergence field in Planck CMB data is detected with more than $1\sigma$ significance at $\ell\lesssim 500$ and more than $14\sigma$ significance at $\ell\gtrsim1500$, yielding an overall $14.8\sigma$ detection. We also show that masking galaxy clusters in CMB data is not sufficient to eliminate the spurious lensing signal, still detecting a non-negligible bias with $5.5\sigma$ significance on cross-correlations with galaxy density fields. Our results emphasize how essential it is to deproject the tSZ effect from CMB maps at the component separation stage and adopt tSZ-free CMB lensing maps for cross-correlations with LSS data.