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

People involved at OAB: Bellazzini, Pancino.

There is now a growing body of observational evidence in favour of an inhomogeneous halo, where the traces 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. The large photometric catalogue for $\sim 500,000$ individual stars in a $1 \times 1 \deg^2$ area centered on the globular cluster M54 has been largely analyzed and is still providing a conspicuous scientific return. Just to summarize the main results obtained from this huge database, we recall: (1) the first identification of the RGB bump in Sgr (Monaco et al. 2002); (2) the detection of the Blue Horizontal Branch population in Sgr and a robust estimate of the fraction of old stars (age larger than 10 Gyr) in the galaxy (Monaco et al., 2003); (3) a new and more reliable distance to the Sgr galaxy obtained from the RGB Tip (Monaco et al. 2004) and (4) the clear demonstration that Sgr is a nucleated dwarf galaxy independently of the presence of M54 at its center (Monaco et al. 2005). The central nucleus of metal-rich stars is clearly resolved and it has been shown to be coincident in postition with M54 but it displays a different profile with respect to the cluster. The catalogue has been recently implemented with B photometry, calibrated by comparison of common stars with the photometry of Rosenberg et al. (2004). Our database has also provided the target list for the spectroscopic survey performed with FLAMES@VLT, within the GTO program of the ITAL-FLAMES consortium. From the analysis of the UVES data (R=40000 spectra) we have obtained abundances of Fe, Ca, Mg and Ti for 15 RGB stars of Sgr. A paper presenting the main results of this study has been recently submitted to A&A (Monaco et al. 2005b). The analysis of the ($\sim 120$) FLAMES spectra is in progress.

In the meantime, we have obtained B,V,I WFI photometry of a new $1 \times 1 \deg^2$ field: Sgr34, located $\sim 2$ deg to the EAST of M54, along the major axis of the galaxy. The Sgr34 catalogue contains $\sim 300000$ stars. A $0.5 \times 0.5 \deg^2$ control field has also been observed and reduced. This allowed a robust and reliable statistical decontamination of the Color Magnitude Diagram of the Sgr34 field (see the cover figure of this section). With this new dataset we will study in detail the star formation history of the Sgr dSph and the population gradients between the central region and the outskirts of the galaxy.

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 (Majewski et al, 2003). As a follow up of the search of globular clusters in the Sgr Stream (Bellazzini et al., 2002, 2003a, 2003b) we have started a programme to study the chemical composition of stars in the Sgr Stream (in collaboration with the Trieste Observatory and with data acquired at TNG-SARG) and a survey of radial velocities in the Stream (data acquired at Loiano).

This research is in collaboration with Ferraro (Univ. of Bologna), and with the INAF-Trieste Observatory group (Monaco, Bonifacio, Girardi, Zaggia) within the Ital-FLAMES Consortium. The photometry of the Sgr34 field is part of the thesis of M. Correnti (Univ. of Bologna).