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SkyMapper Protected Science Project

The Halos of nearby Globular Clusters

Ken Freeman (RSAA), Gary Da Costa (RSAA), Dougal Mackey (RSAA)

The goal of the project is to discover stars in the outer parts of nearby Galactic globular clusters, at and beyond their tidal radii. These rare stars are valuable tracers of the formation and evolution of the clusters. Because they are so rare, wide field imaging in several bands is needed to find them, and thus the project is well suited to the SkyMapper survey. The regions of globular clusters beyond their tidal radii are populated by stars from several possible sources. These include: (a) stars that have evaporated from the cluster through two-body relaxation, mass loss driven by early stellar evolution, or through tidal shocking as the cluster crosses the Galactic disk or passes near the Galactic Centre; (b) stars that belong to a tidally stripped parent dwarf galaxy of which the cluster was the nucleus (e.g., omega Cen, M54) or a former member; and, (c) for globular clusters that may have formed in a local starburst in the manner of NGC2070 in the LMC, the stars that formed in the vicinity of the cluster might still be moving along with the cluster, although not part of it. Such stars may be of interest in the context of current work on the multiple stellar populations present in most globular clusters. In each case the stars may form a halo around the cluster or show leading and trailing tidal tails as is observed in clusters such as Pal 5. The observational aim is to identify stars that are photometrically like the cluster members, using all the SkyMapper band passes for which the photometry is accurate enough to detect cluster-like stars. The analysis templates will come from the distribution of cluster stars in the multi-colour, magnitude space provided by the SkyMapper data. The target list will include relatively nearby clusters to permit spectroscopic follow-up of the extra-tidal population to learn more about their origin. The program will use Main Survey data.