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We present the study of the dynamical status of the galaxy clusterCL1821+643, a rare and intriguing cool-core cluster hosting a giant radio halo.We base our analysis on new spectroscopic data for 129 galaxies acquired at theItalian Telescopio Nazionale Galileo. We also use spectroscopic data availablefrom the literature and photometric data from the Sloan Digital Sky Survey. Weselect 120 cluster member galaxies and compute the cluster redshift <z> ~ 0.296and the global line-of-sight velocity dispersion $\sigma_{\rm V}$ ~ 1100 km/s.The results of our analysis are consistent with CL1821+643 being a massivedynamically relaxed cluster dominated by a big and luminous elliptical at thecentre of the cluster potential well. None of the tests employed to study thecluster galaxies kinematics in the 1D (velocity information), 2D (spatialinformation), and 3D (combined velocity and spatial information) domains isable to detect significant substructures. While this picture is in agreementwith previous results based on X-ray data and on the existence of the centralcool core, we do not find any evidence of a merging process responsible for theradio halo discovered in this cluster. Thus, this radio halo remains an openproblem that raises doubts about our understanding of diffuse radio sources inclusters.

Multi-object spectroscopy of CL1821+643: a dynamically relaxed cluster with a giant radio halo?