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.
We present deep, wide-field g and r photometry of the transition type dwarf galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The data confirm the presence of both very young (<1 Gyr) as well as much older (>5 Gyr) stars. We
study the structural properties of the old and young stellar populations by preferentially selecting either population based on their color and magnitude. The young population is significantly more concentrated than the old population, with half-light radii of 104+-8 and 148+-16 pc respectively, and their centers are slightly offset. Approximately 10% of the total stellar mass is estimated to be represented by the young stellar population. Comparison of the color-magnitude diagram (CMD) with theoretical isochrones as well as numerical CMD-fitting suggest that star formation began over 10 Gyr ago and continued in recent times until at least a few hundred Myr ago. The CMD-fitting results are indicative of two distinct star formation bursts, with a quiescent period around 3 Gyr ago, albeit at low significance. The results are consistent with no metallicity evolution and [Fe/H] ~ -1.5 over the entire age of the system. Finally, the data show little if any sign of tidal distortion of Leo T.
