The Centaurus cluster of galaxies - II. The bimodal velocity structure

The bimodal velocity distribution of the galaxies in the Centaurus cluster ($$\alpha=12^\text h47^\text m,\enspace \delta=-41^\circ$$) is discussed. The mean heliocentric velocities and line-of-sight dispersions of the two main cluster components, within 3^○ of the cluster centre, are 3041, $$586\enspace\text {km}\enspace \text s^{-1}$$ (denoted Cen 30) and 4570, $$262\enspace\text {km}\enspace \text s^{-1}$$ (denoted Cen 45) respectively. Cen 30 is dominant, having about 2.5×the population of Cen 45. Each of the two major components has a large elliptical galaxy lying at its dynamical centre; NGC 4696 in Cen 30 and NGC 4709 in Cen 45. Direct comparisons of the U–V colour–magnitude relationships, the luminosity functions and the galaxian-radius distributions of the core samples of the two components allow their relative distance to be estimated. Provided environmental differences between the components are not important, these methods strongly favour the hypothesis that the two components lie within the same cluster. However, the presence of galaxies with the same velocity as Cen 45 at considerable projected distances from the cluster centre suggests that there is some contamination of Cen 45 by a background system of galaxies that lies in the Hubble flow behind the main cluster. Despite this, we consider the core of Centaurus to be analogous to the stage in White's 700-body simulation where the two major concentrations are about to merge.