Tidal streams in the galactic halo: Evidence for the Sagittarius northern stream or traces of a new nearby dwarf galaxy

Standard cosmology predicts that dwarfs were the first galaxies to be forme d in the universe and that many of them merge afterward to form bigger gala xies such as the Milky Way. This process would have left behind traces such as tidal debris or star streams in the outer halo. We report here the dete ction of a very low density stellar system at 50 +/- 10 kpc from the Galact ic center that could be related to the merger process. It could form part o f the Sagittarius northern stream or, alternatively, could be the trace of a hitherto unknown dwarf galaxy. The dwarf galaxy in Sagittarius, the close st satellite of the Milky Way, is currently being tidally disrupted and is a "living" test for galaxy formation theories. The system found here is 60 degrees away from the center of the Sagittarius galaxy. If it is really ass ociated with this galaxy, it would confirm predictions of dynamical interac tion models indicating that tidal debris from Sagittarius could extend alon g a stream completely enveloping the Milky Way in a polar orbit.