THE CLUSTER OF GALAXIES SC2008-57 (A3667)

We present the results of photometric and spectroscopic observations of the cluster of galaxies SC2008 - 57 (A3667). The observations have resulted in a catalogue with positions and magnitudes for 203 galaxies, complete at b25 = 18.0, and radial velocities for 128 galaxies, 91 per cent complete at b25 = 17.5. The cluster can be classified as type L because its galaxy distribution is highly flattened. It shows two strong concentrations: a main concentration, centred on the cluster brightest galaxy (a D galaxy) and coincident with the peak of X-ray emission, and a substructure around the second brightest galaxy (also a D galaxy). Most of the galaxies in this substructure seem to be bound to the second-brightest galaxy, forming a dynamical subunit inside the cluster. The extreme flattening of the cluster may at least partially be due to the presence of the substructure. The cluster also shows evidence for luminosity segregation, with the brightest galaxies being preferentially found in high galaxy density regions. Most of the luminosity segregation, however, is produced by galaxies associated with the two clumps around the D galaxies, suggesting that dynamical friction is effective in subclusters with low velocity dispersions and may be associated with the formation of D galaxies. The velocity dispersion of SC2008 - 57 is high, about 1200 km s-1, but consistent with the observed X-ray luminosity. The cluster mass, derived using several estimators, is about 2.6 x 10(15) M.. Both the cluster mass and velocity dispersion may be overestimates due to the presence of the substructure.