The non-peculiar velocity dispersion profile of the stellar system ω Centauri

Abstract

We present the results of a survey of radial velocities over a wide region extending from r~10 arcmin out to r~80 arcmin (~1.5 tidal radii) within the massive star cluster omega Centauri. The survey was performed with FLAMES@VLT, to study the velocity dispersion profile in the outer regions of this stellar system. We derived accurate radial velocities for a sample of 2557 newly observed stars, identifying 318 bona-fide cluster red giants. Merging our data with those provided by Pancino et al. (2007), we assembled a final homogeneous sample of 946 cluster members that allowed us to trace the velocity dispersion profile from the center out to r~32 arcmin. The velocity dispersion appears to decrease monotonically over this range, from a central value of sigma_{v}~17.2 Km/s down to a minimum value of sigma_{v}~5.2 Km/s. The observed surface brightness profile, rotation curve, velocity dispersion profile and ellipticity profile are simultaneously well reproduced by a simple dynamical model in which mass follows light, within the classical Newtonian theory of gravitation. The comparison with an N-body model of the evolution of a system mimicking omega Cen during the last 10 orbits into the Galactic potential suggests that (a) the rotation of stars lying in the inner ~20 arcmin of the clusters is not due to the effects of the tidal field of the Milky Way, as hypothized by other authors, and (b) the overall observational scenario is still compatible with the possibility that the outer regions of the cluster are subject to some tidal stirring.