Robustness of Pairwise Kinematic SZ Effect to Optical Cluster Selection Bias

TL;DR

This study investigates how optical cluster selection bias affects the pairwise kSZ measurements, finding that such bias has minimal impact within current uncertainty levels, thus supporting the robustness of kSZ as a cosmological probe.

Contribution

The paper introduces a novel cylindrical count method applied to hydrodynamical simulations to assess optical selection bias effects on pairwise kSZ measurements.

Findings

No significant bias on pairwise kSZ within 16% uncertainty

Minimal impact on pairwise velocities and optical depth

Supports robustness of kSZ measurements against optical selection bias

Abstract

The pairwise Kinematic Sunyaev-Zel'dovich (kSZ) measures both the pairwise motion between massive groups and the amount of gas within them, providing a tracer for the cosmic growth. To interpret the cosmological information in kSZ, it is crucial to understand the optical cluster selection bias on the kSZ observables. Line-of-sight structures that contribute to both the optical observable (e.g. richness) and the kSZ signal can induce a correlation between these two quantities at fixed cluster mass. The selection bias arising from this correlation is a key systematic effect for cosmological analyses and has the potential to resolve the tension between the cosmological constraints from the DES-Y1 cluster counts, lensing, and Planck. In order to test for a kSZ equivalent of such a bias, we adopt an alternative mock richness based on galaxy counts within cylindrical volumes along the line-of-sight. We apply the cylindrical count method to hydrodynamical simulations across a wide range of galaxy selection criteria, assigning richness consistent with DES-Y1 to the mock clusters. We find no significant bias on pairwise kSZ, pairwise velocities, or optical depth, when comparing optically selected clusters to mass-selected halos, within our uncertainty limits of approximately 16, 10, and 8 per cent, respectively.