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 formedin the Universe and that many of them merge afterwards to form bigger galaxiessuch as the Milky Way. This process would have left behind traces such as tidaldebris or star streams in the outer halo. We report here the detection of avery low density stellar system at 50+/- 10 kpc from the Galactic center thatcould be related to the merger process. It could form part of the Sagittariusnorthern stream or, alternatively, could be the trace of a hitherto unknowndwarf galaxy. The dwarf galaxy in Sagittarius, the closest satellite of the Milky Way, iscurrently being tidally disrupted and is a ``living'' test for galaxy formationtheories. The system found here is 60 deg away from the center of theSagittarius galaxy. If it is really associated with this galaxy, it wouldconfirm predictions of dynamical interaction models indicating that tidaldebris from Sagittarius could extend along a stream completely enveloping theMilky Way in a polar orbit.Comment: 14 pages, 3 figures, acepted for publication in Astrophysical Journal Letter