The accreted component of the Galactic Halo: The Sagittarius Dwarf Spheroidal

Involved people at OAB: Bellazzini, Ferraro, Monaco.

There is now a growing body of observational evidences for an inhomogeneous Halo, where the tracks of the slow building up by hierarchical merging of subunits should be still observable and evident. The Sagittarius dwarf Spheroidal Galaxy (Sgr dSph) is the most evident and striking example of a real time accretion event occurring in the Galactic Halo. The main body of Sgr dSph orbits well within the Galactic spheroidal (R$_{GC}\simeq$16 kpc) and shows clear signs of being accreted and distrupted by the Galactic tidal field (e.g., Bellazzini, Ferraro & Buonanno 1999). Thus, the Sgr dSph is (and has been) one of the major contributors to the stellar content of the whole Galactic Halo.

In this framework, we have started a large photometric survey of this disrupting dSph. The final database will consist of V,I photometry and accurate astrometry for five 1$\deg$ fields, sampling different regions of the galaxy that is extended over a huge area of the sky. The data reduction is complete for a first field centered on the globular cluster M 54 (see Figure 1). The large database will provide the necessary insight for the study of the stellar populations and star formation history in this peculiar galaxy, as well as the basis for a detailed study of the chemical composition of its stars to be performed with VLT-FLAMES.

This research is in collaboration with Pancino (Dip. Astr. BO) and with the Trieste Observatory's group (Bonifacio, Molaro) within the ITAL-FLAMES consortium.