Multi-object spectroscopy of CL1821+643: a dynamically relaxed cluster
  with a giant radio halo?
  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 <z> ~ 0.296
and 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 massive
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.
