Robustness of pairwise kinematic Sunyaev–Zel’dovich effect to optical-cluster-selection bias

¹ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1 81679 Munich, Germany
² Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching bei München, Germany
³ Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), Taipei 10617, Taiwan
⁴ Excellence Cluster ORIGINS, Boltzmannstr. 2 85748 Garching, Germany
⁵ Department of Physics and Astronomy, University of Pennsylvania, Philadelphia PA 19104, USA
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1 85741 Garching, Germany
⁷ Department of Astronomy and Astrophysics, University of Chicago, Chicago IL 60637, USA
⁸ Department of Physics, Southern Methodist University, Dallas TX 75205, USA
⁹ European Southern Observatory, Karl Schwarzschildstrasse 2, 85748 Garching bei München, Germany
¹⁰ Department of Physics, University of Arizona, 1118 East Fourth Street Tucson AZ 85721, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 May 2025
Accepted: 1 April 2026

Abstract

The pairwise kinematic Sunyaev–Zel’dovich (kSZ) effect measures both the pairwise motion between galaxy groups and clusters and the amount of gas within them, providing a tracer for cosmic growth. To interpret the cosmological information in the kSZ measurements, 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 cosmological signal can induce a correlation between these two quantities at a fixed cluster mass. The selection bias arising from this correlation is a key systematic effect for cosmological analyses. For cosmological observables such as cluster abundance and weak lensing, controlling this selection bias may help explain the tension between the DES-Y1 results and the Planck constraints. In order to test for a kSZ effect equivalent of such a bias, we adopted an alternative mock richness based on galaxy counts within cylindrical volumes along the line of sight. We applied 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. When comparing optically selected clusters to mass-selected halos, we find no significant bias on pairwise kSZ signals, pairwise velocities, or optical depth within our uncertainty limits of approximately 16, 10, and 8%, respectively.