A Spectroscopic Survey for Omega Centauri Members at and beyond the Cluster Tidal Radius

Abstract

We have used the two-degree field (2dF) multi-fibre spectrograph of the Anglo-Australian Telescope to search for candidate members of the unusual globular cluster omega Centauri at and beyond the cluster tidal radius. Velocities with an accuracy of ~10 kms-1 were obtained for 4105 stars selected to lie in the vicinity of the lower giant branch in the cluster colour-magnitude diagram and which cover an area on the sky of ~2.4x3.9 deg2 centered on the cluster. Within the velocity interval 190-270 kms-1, the cluster member candidates have a steeply declining surface density distribution consistent with the adopted tidal radius of 57`. For stars in the sample beyond the tidal radius, an analysis of line-strengths from the spectra, as well as radial velocities, identifies only six stars as possible candidates for extra-tidal association with the cluster. If all six of these stars are indeed related to the cluster, then a maximum of 0.7 +/- 0.2 per cent of the total cluster mass is contained in the region between one and two tidal radii. Given this limit, we conclude that there is no compelling evidence for any significant extra-tidal population in omega Cen. The effects of tidal shocks on the outer parts of the cluster are consistent with this limit. Theories for the origin of omega Cen frequently suggest that the cluster is the former nucleus of a tidally stripped dwarf galaxy. Our results require that the stripping process must have been largely complete at early epochs, consistent with dynamical models of the process. The stripped former dwarf galaxy stars are therefore now widely distributed around the Galaxy.

29 pages, 9 figures, accepted for publication in AJ

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• RAVE stars tidally stripped/ejected from $\omega$ Centauri globular cluster
• Period Change Similarities among the RR Lyrae Variables in Oosterhoff I and Oosterhoff II Globular Systems
• The dynamics of the outer parts of omega Centauri
• The not so simple stellar system Omega Cen. II. Evidence in support of a merging scenario
• STREGA: STRucture and Evolution of the GAlaxy. I. Survey Overview and First Results
• Detecting Globular Cluster Tidal Extensions with Bayesian Inference: I. Analysis of $\omega$ Centauri with Gaia EDR3
• The subgiant branch of omega Cen seen through high-resolution spectroscopy. II. The most metal-rich population
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• The search for extratidal star candidates around Galactic globular clusters NGC 2808, NGC 6266, and NGC 6397 with {\textit{Gaia}} DR2 astrometry
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