We present the study of the dynamical status of the galaxy cluster CL1821+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 the Italian Telescopio Nazionale Galileo .
We also use spectroscopic data available from the literature and photometric data from the Sloan Digital Sky Survey .
We select 120 cluster member galaxies and compute the cluster redshift \left < z \right > \sim 0.296 and the global line-of-sight velocity dispersion \sigma _ { V } \sim 1100 km s ^ { -1 } .
The results of our analysis are consistent with CL1821+643 being a massive ( M > 10 ^ { 15 } M _ { \odot } ) dynamically relaxed cluster dominated by a big and luminous elliptical at the centre of the cluster potential well .
None of the tests employed to study the cluster galaxies kinematics in the 1D ( velocity information ) , 2D ( spatial information ) , and 3D ( combined velocity and spatial information ) domains is able to detect significant substructures .
While this picture is in agreement with previous results based on X-ray data and on the existence of the central cool core , we do not find any evidence of a merging process responsible for the radio halo discovered in this cluster .
Thus , this radio halo remains an open problem that raises doubts about our understanding of diffuse radio sources in clusters .