The accreted component of the Galactic disc: the Canis Major dwarf spheroidal

The accretion of the Sagittarius galaxy is currently the only strong evidence that the Milky Way is absorbing satellite galaxies. However, the ring-like structure that has recently been discovered around the galaxy could be the consequence of another such event. First discovered as an overdensity of blue stars in the Sloan Digital Sky Survey (Newberg et al. 2002), the ``ring'' has since been probed using the INT WFS photometry (Ibata et al. 2003), the SDSS spectrometry and photometry (Yanni et al. 2003) and the 2MASS M giants (Rocha-Pinto et al. 2003). Surrounding the Galactic disk, its galactocentric distance ranges from $\sim 15$ kpc to $\sim 20$ kpc in fields taken within $30\deg$ of the Galactic plane and for $227\deg>l>122\deg$ . It has been proposed (Ibata et al. 2003; Yanni et al. 2003; Helmi et al. 2003) that the structure could be the stellar stream stripped away by the Milky Way tides from a satellite galaxy whose orbital plane is close to the plane of the galaxy.

While studying the large scale structure of the ring using M giant stars from the 2MASS database, we have discovered a large overdensity of stars in the southern Galactic hemisphere (Martin et al. 2003; hereafter M04), in the direction of the Canis Major constellation, approximately enclosed between

and

and $-20^o\le b \le 0^o$ (see Fig. 3). The elliptical shape of this overdensity, its overall structure and its spatial coincidence with a small compact group of globular clusters (Bellazzini et al. 2003c) strongly suggest that we have discovered a previously unknown dwarf galaxy, whose progressive disruption by the Galactic tidal field is probably at the origin of the ring (see also Frinchaboy et al. 2004). The newly discovered stellar systems turned out to be the nearest of all the Milky Way satellites, lying at 8-10 kpc (i.e., 26000-32600 light years).

**Figure 3:** Pixel maps of the distribution of the differences in M-giant starcounts between the two Galactic hemispheres, for different ranges of Galactocentric distance modulus. Note the large elliptical overdensity in the southern hemisphere at , particularly evident in the middle panel. The northern part of the Ring is also visible toward the Galactic Anticenter.
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We searched the literature for wide field photometry of open clusters located in the foreground of the Canis Major object (Bellazzini et al. 2003, hereafter B03) and we detected its main sequence (MS) stars in several directions within the above quoted range. In particular, from the ESO-WFI photometry of the cluster NGC 2477 by Momany et al. (2001), we obtained the first optical CMD of the system, fully confirming the results of M04. Finally, we have found very recent WFI observations of a field located at

, just $\sim 5$ degrees from the center of the newly discovered structure. The data further confirm our discovery of a previously unknown stellar system, the Canis Major galaxy (CMa), a new nearby dwarf satellite of the Milky Way.

The reduction of the data of a large spectroscopic campaign performed at the 2dF-AAT is currently in progress. Preliminary results suggest that the Canis Major system has a distinct velocity with respect to ordinary disc stars and has a velocity dispersion typical of local dwarf spheroidals ( $\sigma \sim 10$ km s $^{-1}$ ).

The ``in-plane'' orbit of the newly discovered relic strongly suggests that we are witnessing - for the first time - a phenomenon related to the last phases of the hierarchical assembly of the thick disc. The discovery had a large echo on the scientific press .