The Centaurus I cluster of galaxies - An extreme case of contamination?

Difficulties in the determination of the true membership of systems for virial calculations are illustrated by the resolution of the Centaurus I cluster of galaxies into two distinct velocity systems based on redshift data. The radial velocities of 149 galaxies with velocities less than 6000 km/sec surrounding the cluster center are shown to exhibit a bimodal distribution corresponding to one group of 106 members with a mean heliocentric velocity of 3179 km/sec and a velocity dispersion of 517 km/sec and a second group of 40 at a mean velocity of 4738 km/sec and dispersion of 255 km/sec. Calculations of the difference in the Hubble distance moduli of the two systems are found to be compatible with clusters of equal population and 1-magnitude luminosity differences, and with unequal populations at similar distances. In either case, the existence of two systems leads to a substantially reduced virial mass. Evidence for the presence of additional velocity systems is also pointed out, and the need for the accurate assessment of membership contamination and substructure in the derivation of cluster parameters is emphasized.