CLASH-VLT: The variance in the velocity anisotropy profiles of galaxy clusters

¹ INAF-Osservatorio Astronomico di Trieste Via G. B. Tiepolo 11 34143 Trieste, Italy
² IFPU-Institute for Fundamental Physics of the Universe Via Beirut 2 34014 Trieste, Italy
³ Dipartimento di Fisica, Università degli Studi di Milano Via Celoria 16 I-20133 Milano, Italy
⁴ Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano-Bicocca Piazza della Scienza 3 I-20126 Milano, Italy
⁵ Department of Physics and Earth Sciences, University of Ferrara, Via G. Saragat 1 44122 Ferrara, Italy
⁶ Dipartimento di Fisica E. R. Caianiello, Università Degli Studi di Salerno Via Giovanni Paolo II I-84084 Fisciano (SA), Italy
⁷ INAF – Osservatorio Astronomico di Capodimonte Via Moiariello 16 I-80131 Napoli, Italy
⁸ IATE – Instituto de Astronomía Teórica y Experimental, Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET), Universidad Nacional de Córdoba Laprida 854 X5000BGR Córdoba, Argentina
⁹ INAF – IASF Milano Via A. Corti 12 I-20133 Milano, Italy
¹⁰ Dipartimento di Fisica dell’Università degli Studi di Trieste – Sezione di Astronomia Via Tiepolo 11 I-34143 Trieste, Italy
¹¹ University Observatory, Ludwig-Maximilians University Munich Scheinerstrasse 1 81679 Munich, Germany
¹² Centro de Astrobiología (CAB), CSIC-INTA Ctra. de Ajalvir km 4 Torrejón de Ardoz 28850 Madrid, Spain

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Received: 8 May 2025
Accepted: 31 January 2026

Abstract

Context. The velocity anisotropy profiles, β(r), of galaxy clusters are directly related to the shape of the orbits of their member galaxies. Knowledge of β(r) is important in order to understand the assembly process of clusters and the evolutionary processes of their galaxies, and to improve the determination of cluster masses based on cluster kinematics.

Aims. We determined the β(r) of nine massive clusters at redshift 0.19 ≤ z ≤ 0.45 from the CLASH-VLT dataset, with ≃150–950 spectroscopic members each, to understand how much cluster-to-cluster variance exists in the β(r) of different clusters and what the main driver of this variance is.

Methods. We selected spectroscopic cluster members with the CLUMPS algorithm calibrated on cosmological simulations. We applied the MAMPOSSt code to the distribution of cluster members in projected phase-space to constrain the cluster mass profile, M(r), using priors derived from a previous gravitational lensing analysis. Given the MAMPOSSt best-fit solution for M(r), we then solved the inversion of the Jeans equation to determine β(r) without assumptions of its functional form. We also ran the DS+ code to identify subclusters and characterize the dynamical status of our clusters.

Results. The average ⟨β(r)⟩ is slightly radial; the anisotropy increases from β ≃ 0.2 at the cluster center to β ≃ 0.5 at the virial radius. There is substantial variance in the β(r) of the individual clusters that cannot be entirely accounted for by the observational uncertainties. Clusters of lower mass and with a low concentration per given mass have more tangential β(r) profiles. A comparison with previous works in the literature suggests that orbits are more radial in clusters at higher z. A comparison with cluster-sized halos in cosmological hydrodynamical simulations indicates a very good agreement for the average ⟨β(r)⟩, but a smaller variance in the profiles than observed.

Conclusions. Massive clusters cannot be characterized by a unique universal β(r). The orbital distribution of cluster galaxies carries information on the merging history of the cluster.