The elliptical configuration of the X-ray-emitting intracluster medium

Department of Astronomy, Yale University, P. O. Box 208101 New Haven, CT 06520-8101, USA

^⋆ Corresponding author: graziella.zinn@yale.edu

Received: 8 July 2025
Accepted: 12 October 2025

Abstract

Context. The morphology of the hot gas X-ray images within galaxy clusters provides indications about the formation, populations, mergers, and evolution of clusters in connection with their dynamical state.

Aims. The aim of this investigation is to search for a link between one or more basic morphological parameters describing the elliptical configuration of intracluster medium (ICM) X-ray contours and the dynamical state of the parent cluster.

Methods. We used the 2021 X-CLASS Survey Catalogue of X-ray-detected galaxy clusters. We selected the sample of 982 spectroscopically confirmed clusters, and visually inspected the shape and orientation of their hot gas X-ray contours. We recognized a variety of morphological structures that could lead to a certain dynamical state. A more detailed analysis was carried out by fitting ellipses to the X-ray digitized contours of a subsample of 105 clusters, already classified in the literature as dynamically relaxed, dynamically non-relaxed, or dynamically intermediate.

Results. Some correlations have been found among our measurements of ellipticity (ϵ) for ICM X-ray contours and the classified dynamical state of the parent cluster. The majority of the relaxed clusters present an increase in ellipticity outward from the cluster center; exceptions include a conspicuous number of clusters with essentially ICM constant ellipticity. In contrast, most of the non-relaxed clusters have ellipticity profiles decreasing outward, with fewer exceptions. Moreover, the total variation in ellipticity (Δϵ) tends to be smaller for the X-ray contours of relaxed clusters than for those of non-relaxed clusters, including maximum ellipticity values. Intermediate clusters overlap with both groups. Similar results are found by applying the selected morphological parameters to ICM X-ray contours available in the literature from other X-ray observations.