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

People involved at OAB: Bellazzini.

The accretion of the Sagittarius galaxy is currently the only strong evidence that the Milky Way is accreting 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). The galactocentric distance of this ring surrounding the Galactic disk 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, between $l=220^o$ and $l=260^o$ and $-20^o\le b \le 0^o$. 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 system is the nearest of all the Milky Way satellites, lying at 8-10 kpc (i.e., 26000-32600 light years).

Deep B,V photometry of a field near the center of CMa has been presented in Bellazzini et al. (2004). The CMD of this field reveals the presence of a well defined and narrow Main Sequence typical of an old-intermediate age metal-rich population. This feature is not predicted by state-of-the-art Galactic models, confirming the presence of an extra population in this region of the Milky Way. In Martin et al. (2004) we have presented the first results of a large spectroscopic survey performed with the 2dF spectrograph at the AAT telescope. In this paper it has been shown that CMa has a systemic velocity significantly different from that expected for a population belonging to the Galactic Disc and a low velocity dispersion ($\sim 10$ km/s), quite typical for a dwarf galaxy. A new distance estimate based on the RGB Tip has also been obtained ( $D_{\odot}\simeq 7.2$ kpc) and strong arguments have been presented against the hypothesis that CMa may be associated with the warp of the Galactic disc.

At present we are carrying out an extensive study of the structure of CMa using RC stars as tracers (instead of M-giants as in previous works) and the huge spectroscopic database ($\sim 2000$ stars) is being fully analyzed.

This research is in collaboration with: L. Monaco (INAF-Trieste Obs.), R. Ibata, N. Martin (Strasbourg Obs.), G. Lewis, B. Conn (Sidney University), M.J. Irwin (Cambridge - UK).