Structure and evolution of galaxy clusters: Internal dynamics of ABCG 209 at z similar to 0.21

We study the internal dynamics of the rich galaxy cluster ABGC 209 on the basis of new spectroscopic and photometric data. The distribution in redshift shows that ABCG 209 is a well isolated peak of 112 detected member galaxies at z = 0.209, characterised by a high value of the line-of-sight velocity dispersion, sigma(v) = 1250-1400 km s(-1), on the whole observed area (1 h(-1) Mpc from the cluster center), that leads to a virial mass of M = 1.6-2.2 x 10(15)h(-1) M. within the virial radius, assuming the dynamical equilibrium. The presence of a velocity gradient in the velocity field, the elongation in the spatial distribution of the colour-selected likely cluster members, the elongation of the X-ray contour levels in the Chandra image, and the elongation of cD galaxy show that ABCG 209 is characterised by a preferential NW-SE direction. We also find a significant deviation of the velocity distribution from a Gaussian, and relevant evidence of substructure and dynamical segregation. All these facts show that ABCG 209 is a strongly evolving cluster, possibly in an advanced phase of merging.