The Centaurus cluster of galaxies - III. Its structure and the distribution of the different galaxy types

Abstract

We have previously shown that the Centaurus cluster (⁠|$\alpha=12^\text h47^\text m,\enspace \delta=-41$|°) is composed of two velocity components, Cen 30 (mean velocity 3000kms−1) and Cen 45 (mean velocity 4500kms−1), which very probably lie within one cluster. In this paper we describe the internal structure of the cluster and discuss the spatial and velocity distributions of the different galaxy types within the cluster. Centaurus possesses extensive substructure, with several recognizable features within the Cen 30 and Cen 45 components. In the outer parts of the cluster, groups of galaxies have been identified. The most prominent, the ‘Western Branch’, is a flattened structure that extends westwards and slightly northwards of the cluster centre. This feature is made up of equal contributions from the two velocity systems. Another notable group, the NGC 4930 group, lies 2.°5 to the east of the cluster. It is a loose aggregate consisting almost entirely of spiral galaxies, with a small velocity dispersion and a mean velocity |$\sim490\text {kms}^{-1}$| lower than the mean velocity of the dominant Cen 30 component. Other recognized groups are more compact and are often dominated by a large elliptical galaxy, e.g. the NGC 4373 group and the NGC 4767 group. We discuss the substructure in clusters generally and support the view that it is a common feature of most galaxy clusters. The morphology–density relationship is very striking within the cluster, with the early-type galaxies dominating the cluster core. Although the spiral galaxy distribution shows little central concentration, several distinct groups, e.g. the NGC 4930 group, are apparent. Unlike the Virgo cluster, we find that for Centaurus the velocity dispersions of the different galaxy types are similar. However, the spiral galaxies have a more clumpy velocity distribution which we interpret as additional evidence that spiral-dominated groups of galaxies exist within Centaurus. The presence of these spiral-dominated groups ensures that the infall rate of spirals into the cluster core will be very irregular. This guarantees that at some epoch the spirals would be observed to possess a larger velocity dispersion than the elliptical and S0 galaxies. It is concluded that a cluster's population mix depends on the epoch at which the main body of the cluster was formed.