Multi-object spectroscopy of CL1821+643

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}_V_~1100km/s. The results of our analysis are consistent with CL1821+643 being a massive (M>10^15^M_{sun}) 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.