Inference of the morphology and dynamical state of nearby Planck-SZ galaxy clusters with Zernike polynomials

¹ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy
² Physics Department “Ettore Pancini”, Università degli studi di Napoli “Federico II”, Via Cintia 21, I-80126 Napoli, Italy
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁴ Departamento de Física Teórica, Módulo 8, Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
⁵ Centro de Investigación Avanzada en Física Fundamental (CIAFF), Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
⁶ University of Lyon, UCB Lyon 1, CNRS/IN2P3, IP2I Lyon, France

^⋆ Corresponding author: valentina.capalbo@uniroma1.it

Received: 17 October 2024
Accepted: 7 April 2025

Abstract

Aims. We analyse maps of the Sunyaev–Zel’dovich (SZ) signal of local galaxy clusters (z<0.1) observed by the Planck satellite to classify their dynamical state via morphological features.

Methods. We applied a novel method, which was recently employed on mock SZ images generated from hydrodynamical simulated galaxy clusters in THE THREE HUNDRED (THE300) project to study the morphology of the cluster maps. In this paper, we report our results following its first application on real data. The method consists of modelling the images with a set of orthogonal functions defined on circular apertures, namely, the Zernike polynomials. From the fit we computed a single parameter, 𝒞, that quantifies the morphological features present in each image. The link between the morphology of 2D images and the dynamical state of the galaxy clusters is well known, even if it is not obvious. We used mock Planck-like Compton parameter maps generated for THE300 clusters to validate our morphological analysis. These clusters are, in fact, properly classified for their dynamical state with the relaxation parameter, χ, by exploiting 3D information from the simulations.

Results. We find a mild linear correlation of ∼38% between 𝒞 and χ for THE300 clusters, mainly affected by the noise present in the maps. To obtain a proper dynamical-state classification for the Planck clusters, we exploited the conversion from the 𝒞 parameter derived in each Planck map in χ. A fraction of the order of 63% of relaxed clusters has been estimated in the selected Planck sample. Our classification was then compared with those of previous works that have attempted to evaluate, with different indicators and/or other wavelengths, the dynamical state of the same Planck objects. We find an agreement with these other works to be greater than 58%.