The dynamics of the outer parts of omega Centauri

TL;DR

This study uses spectroscopic data to measure radial velocities in omega Centauri's outer regions, confirming a constant velocity dispersion likely caused by external tidal effects rather than dark matter.

Contribution

It provides new precise velocity measurements for outer omega Centauri members, expanding the dataset and analyzing the velocity dispersion profile in the context of external influences.

Findings

Velocity dispersion remains constant at ~6.5 km/s in outer regions.

External tidal effects likely cause the observed velocity dispersion.

No need to invoke dark matter or alternative gravity theories.

Abstract

The multi-object fibre-fed spectrograph AAOmega at the Anglo-Australian Telescope has been used to establish and measure accurate (<1 kms-1) radial velocities for a new sample of members in the outer parts of the stellar system omega Centauri. The new sample more than doubles the number of known members with precise velocities that lie between 25' and 45' from the cluster center. Combining this sample with earlier work confirms that the line-of-sight velocity dispersion of omega Cen remains approximately constant at ~6.5 kms-1 in the outer parts of the cluster, which contain only a small fraction of the total cluster stellar mass. It is argued that the approximately constant velocity dispersion in the outer regions is most likely a consequence of external influences, such as the tidal shock heating that occurs each time omega Cen crosses the Galactic plane. There is therefore no requirement to invoke dark matter or non-standard gravitational theories.