CHEX-MATE: The impact of triaxiality and orientation on Planck SZ cluster selection and weak lensing mass measurements

¹ California Institute of Technology, 1200 East California Boulevard, Pasadena, California, USA
² IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
³ Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, USA
⁴ Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
⁵ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, I-40129 Bologna, Italy
⁷ INFN, Sezione di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy
⁸ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, 20133 Milano, Italy
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ IRAP, CNRS, Université de Toulouse, CNES, Toulouse, France
¹¹ INFN-Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy
¹² Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, 41125 Modena, Italy
¹³ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185 Rome, Italy
¹⁴ INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34131 Trieste, Italy
¹⁵ IFPU, Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
¹⁶ Department of Physics; University of Michigan, Ann Arbor, MI 48109, USA
¹⁷ Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, CH-1290 Versoix, Switzerland
¹⁸ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
¹⁹ HH Wills Physics Laboratory, University of Bristol, Bristol, UK

^⋆ Corresponding author.

Received: 29 May 2025
Accepted: 10 June 2025

Abstract

Context. Galaxy cluster abundance measurements are a valuable tool for constraining cosmological parameters, such as the mass density (Ω_m) and the density fluctuation amplitude (σ₈). Wide-area surveys detect clusters based on observables, such as the total integrated Sunyaev-Zel’dovich effect signal (Y_SZ) in the case of Planck. Quantifying the survey selection function is necessary for cosmological analyses, with completeness representing the probability of detecting a cluster as a function of its intrinsic properties, such as Y_SZ and an angular scale θ₅₀₀.

Aims. We determine the completeness of the Planck-selected CHEX-MATE cluster catalog using mock observations of clusters with triaxial shapes and random orientations, with physically-motivated distributions of axial ratios. From these mocks, we derive the distribution of shapes and orientations of the detected clusters, along with any associated bias in weak-lensing-derived mass (M_WL) due to this orientation-dependent selection (denoted as 1 − b_χ).

Methods. Employing a Monte Carlo method, we injected triaxial cluster profiles into random positions within the Planck all-sky maps and subsequently determined the completeness as a function of both geometry and SZ brightness. This result was then used to generate 1000 mock CHEX-MATE cluster catalogs. We computed M_WL for these mock CHEX-MATE clusters and for equal-sized samples of randomly selected clusters with similar mass and redshift distributions.

Results. Cluster orientation impacts completeness, with a higher probability of detecting clusters elongated along the line of sight (LOS). This leads to 1 − b_χ values of 0−4% for CHEX-MATE clusters relative to a random population. The largest increase in M_WL is observed in the lowest-mass objects, which are most impacted by orientation-related selection bias.

Conclusions. Clusters in Planck SZ-selected catalogs are preferentially elongated along the LOS and have an average bias in M_WL relative to randomly selected cluster samples. This bias is relevant for upcoming SZ surveys such as CMB-S4, and should be considered for surveys utilizing other probes for cluster detection, such as Euclid.