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

People involved at OAB: Bellazzini, Monaco, Pancino.

There is now a growing body of observational evidence in favour of an inhomogeneous halo, where the tracks of the slow building up by hierarchical merging of sub-units should be still observable. The Sagittarius dwarf Spheroidal Galaxy (Sgr dSph; Ibata et al. 1994) 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 spheroid (R$_{GC}\simeq16$ kpc) and shows clear signs of being accreted and disrupted by the Galactic tidal field. 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. At present we have assembled a catalogue comprising $V$, $I$ photometry and accurate astrometry (to within $\pm 0.2$ arcsec) for $\sim 500,000$ individual stars in a $1 \times 1 \deg^2$ area centered on the globular cluster M54, which coincides with the main density peak of the Sgr galaxy. Near-Infrared $J$, $H$, $K$ magnitudes for many thousands of stars have also been obtained from the Point Source catalogue of the 2MASS survey. This large database provided the basis for a detailed study of the chemical composition of Sgr stars to be performed with FLAMES@VLT, within the GTO program of the ITAL-FLAMES consortium.

Meanwhile, the analysis of the photometric database (Monaco et al. 2002) is ongoing. We have detected blue horizontal branch stars unequivocally associated with the Sgr galaxy, thus demonstrating that the system contains also old and metal-poor stars (Monaco et al. 2003). We have also shown that there is a nucleus at the center of Sgr, coincident in position with the globular cluster M54 but populated by stars that cannot belong to the cluster (much more metal-rich; Monaco et al. 2004). This finding shows that Sgr was indeed a nucleated galaxy, independently of the presence of M54.

It is now established that the disruption of the Sgr dSph left a track of stars (torn away from the original Sgr dSph) that forms a long-lived structure, following the past orbital path of the galaxy, i.e. the Sgr Stream (Ibata et al. 2001). We have started a program to search for possible relations between known globular clusters and the Sgr Stream. As a first important result we have found that the orbit of the Sgr galaxy is a preferential locus in the phase-space for the globular clusters in the range $10 \le R_{GC} \le 40$ kpc (Bellazzini, Ferraro & Ibata 2003). We have demonstrated that the observed phase-space clustering is statistically significant, thus indicating that Sgr has left behind in the Galactic halo not only a stream of stars but also some globular clusters. A first pilot search of Sgr Stream stars around candidate Stream clusters brought to the observational confirmation of two clusters as bona-fide members of the Sgr stream, i.e. Pal 12 and NGC 4147.

This research is in collaboration with Ferraro (Astron. Dept., University of Bologna), and with the INAF-Trieste Observatory group (Bonifacio, Molaro, Zaggia) within the ITAL-FLAMES Consortium. The research on the Sagittarius Stream is carried out in collaboration with R. Ibata (Observatoire de Strasbourg).