Velocity structure of the dwarf galaxy population in the Centaurus cluster

Based on the photometric survey of the inner region oftheCentauruscluster(Jerjen&Dressler1997a)wemeasured redshifts for a deep, surface brightness limited sample of galax- ies using the MEFOS multibre spectrograph at the ESO 3.6m telescope. With the new data set radial velocities for 120 cen- trallylocatedclustermembersbecomeavailablewhichisequiv- alent to 78% of all known cluster galaxies in the region brighter than BT=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 verydifferentcharacteristicsofthevelocitydistributionsforthe 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 314898kms 1 . The re- markable coincidence of this velocity with the mean velocity of Cen30 and the redshift of NGC4696 in particular strongly suggests a connection of the dE&dS0s to the gravitational cen- tre 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 NGC4696. The dy- namical 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 remainsunclear.WeshowthatthepoornessofCen45represents an intrinsic problem which makes it difcult to approach this question.