$\omega$ Centauri

People involved at OAB: Bellazzini, Cacciari, Pancino, Romano, Tosi

$\omega$ Centauri is the largest ( $M=2.9\,10^6 M_{\odot}$, Merrit et al. 1997), brightest cluster in the Galactic Halo, and surely the most peculiar one in terms of structure, kinematics and stellar contents. It is in fact the only GC which shows undisputed variations in the chemical content of its stars. From this point of view, $\omega$ Cen could be considered a bridge system between genuine globulars, which are unable to retain the gas ejected by their former massive stars, and dwarf galaxies, which are the least massive self-enriching stellar systems known.

Figure: Comparison between the best-fitting Wilson (1975) model (solid lines) and (a) the projected surface brightness (from Trager et al. 1995), (b) the ellipticity profile (from Geyer et al. 1983), (c) the rotation curve and (d) the velocity dispersion profile presented in this paper. In panel (c), the $1\sigma$, $2\sigma$ and $3\sigma$ contours predicted by the model are also shown as dashed lines.

Our group started working on this fascinating cluster years ago (Pancino et al. 2000) producing results on many aspects such as photometry, kinematics, chemistry, in a series of 15 papers in international journals. In 2009, we focussed mainly on the few following topics: (a) a very large kinematic survey (radial velocities for $\sim 2600$ stars from FLAMES spectra) to study the velocity dispersion curve at very large distances from the cluster centre (Sollima et al. 2009, see Fig. ); (b) a more detailed modelling of the chemical evolution of the system: the first paper dealing with the helium content has been published (Romano et al. 2010); (c) a dedicated search for the tidal debris of $\omega$ Cen in the solar neighbourhood, using both kinematical and chemical information, for which we have acquired most of the needed spectra with UVES@VLT and SARG@TNG. This research is in collaboration with A. Sollima (IAC tenerife), R. Smart (OATO), and F.R. Ferraro (UniBO).