Velocity structure of the dwarf galaxy population in the Centaurus cluster

Abstract

Based on the photometric survey of the inner region of the Centaurus cluster (Jerjen & Dressler 1997a) we measured redshifts for a deep, surface brightness limited sample of galaxies using the MEFOS multifibre spectrograph at the ESO 3.6m telescope. With the new data set radial velocities for 120 centrally located cluster members become available which is equivalent to 78% of all known cluster galaxies in the region brighter than B_T=18.5. The relevant aspect of this investigation is that new redshifts for 32 dwarf galaxies have been measured, rising the total number to 48. We investigate the prominent bimodal velocity distribution of Centaurus in more detail, discussing the very different characteristics of the velocity distributions for the main Hubble types E&S0, spirals, Im&BCD, and dE&dS0. The nucleated, bright dwarf ellipticals are the only galaxies with a Gaussian-like distribution centred at 3148+/-98 km/s. The remarkable coincidence of this velocity with the mean velocity of Cen30 and the redshift of NGC 4696 in particular strongly suggests a connection of the dE&dS0s to the gravitational centre of the Centaurus cluster and/or to the cluster dominant E galaxy. The application of statistical tests reveals the existence of a population dwarf galaxies bound to NGC 4696. The dynamical parameters for the two velocity components suggest that Cen30 is the real Centaurus cluster whereas Cen45 can only be a loosely bound group of galaxies. This conclusion is followed up with a type-mixture analysis. All results are fully consistent with the cluster-group scenario. Whether Cen45 is merging with the cluster or is located in the close background remains unclear. We show that the poorness of Cen45 represents an intrinsic problem which makes it difficult to approach this question.