Recent wide-field photometric surveys ( Lee et al .
; Pancino et al . )
have shown the existence of a previously unknown metal-rich ( [ Fe / H ] \simeq - 0.6 ) stellar population in the galactic globular cluster \omega Centauri .
The discovery of this new component , which comprises only a small percentage ( \sim 5 \% ) of the entire cluster population , has added a new piece to the already puzzling picture of the star formation and chemical evolution of this stellar system .
In this Letter we show that stars belonging to the newly discovered metal-rich population have a coherent bulk motion with respect to the other cluster stars , thus demonstrating that they formed in an independent self-gravitating stellar system .
This is the first clear-cut evidence that extreme metal-rich stars were part of a small stellar system ( a satellite of \omega Centauri ? )
that has been accreted by the main body of the cluster .
In this case , we are witnessing an in vivo example of hierarchical merging on the sub-galactic scale .