The accreted component of the Galactic Halo:
The Sagittarius Dwarf Spheroidal and other large
structures

People involved at OAB: Bellazzini, Correnti.

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 (Bell et al. 2007). 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 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.

Our research on the Sagittarius galaxy and its Stream is continuously ongoing with a constant production of new published results (Monaco et al. 2002, 2003, 2004, 2005a,b, 2007; Bellazzini et al. 1999a,b, 2003a,b, 2006a,b; Correnti et al. 2007). A high-resolution spectroscopy survey of stars in the various branches of the Sgr Stream is also ongoing and is providing the first interesting results (Monaco et al. 2007).

Figure: Velocity dispersion profile of M54 stars. The upper panel shows the $V_r$ distribution as a function of distance from the cluster centre for individual stars of M54. Only stars plotted as dots encircled by open pentagons are retained for the computation of $\sigma$ in the various radial bins: small dots alone are stars rejected only because they are ``local'' $3\sigma$ outliers of the bins, crosses are stars that would have been rejected also as $3\sigma$ outliers of the whole Sgr,N + M54 sample (the global $\pm 3\sigma$ range is enclosed by the long-dashed lines). The vertical lines display the adopted independent bins, of variable size. The global mean is marked by the continuous horizontal thick line. The lower panel displays the actual Velocity dispersion profile. The large filled pentagons are the dispersions estimated in the corresponding bins displayed in the upper panel, with their bootstrapped errors. The number of stars per bin is also reported below the points. The small filled pentagons are the estimates in the additional, partially overlapping, bins. The open pentagon is the estimate of $\sigma$ at the centre of M54, obtained by Illingworth (1976) from integrated spectroscopy. From Bellazzini et al. (2008).

A huge kinematical study of the nucleus of Sgr and of the giant cluster M54 that resides in the nucleus itself, including the analysis of the radial velocity for 1152 stars observed with VLT-FLAMES and Keck-DEIMOS, as well as a suite of N-body simulations has been concluded, and a paper has been published (Bellazzini et al. 2008; see Fig. 4). The results presented in this paper open a new window on the process of galaxy nucleation. A follow-up search for a central intermediate mass Black Hole is ongoing.

M. Correnti, within his PhD project, is using SDSS data to take accurate measures of the northern branch of the Sgr tidal Stream. Preliminary results of this study were presented in Correnti et al. (2007, 2008). During this project we serendipitously re-discovered the stellar system Bootes III (see Fig. 5), showing that it hosts also a Red Clump population (Correnti et al. 2009). The system has been later spectroscopically confirmed to be a real ultra-faint dwarf spheroidal galaxy (Carlin et al. 2009).

Figure: Adaptive density map of the Bootes III dwarf galaxy using Red Clump stars as tracers, following Correnti et al. (2009).

An extension to the analysis of the whole CMa/Monoceros system, using Main Sequence stars as tracers, is in progress and is expected to provide clear indications on the actual nature of this huge substructure in the outer Galactic Disc (Conn et al. 2007, 2008).

This research is in collaboration with F. Ferraro (Univ. of Bologna), L. Monaco (ESO, Chile), R. Ibata (Obs. Strasbourg, France), N. Martin (MPI, Germany), M. Irwin (Cambridge, UK), D. Mackey (Edinburgh Univ., UK), and S. Chapman (Caltech, USA). Matteo Correnti's PhD project is focussed on the study of galactic relics, including Sgr. This research was partly supported by INAF-PRIN05 grant n. CRA 1.06.08.02 (PI: M. Bellazzini).