Tracing out the northern stream of the Sagittarius dwarf galaxy with color- magnitude diagram techniques

Standard cosmology predicts that dwarfs were the first galaxies to be formed in the Universe and that many of them merge afterwards to form bigger galaxies such as the Milky Way. This process would have left behind traces such as tidal debris or tidal streams in the outer halo. We report here the detection of two new tidal debris of the northern stream of the Sagittarius dwarf galaxy, based in the analysis of wide field, deep color- magnitude diagrams. These detections provide strong observational evidence that the stripped debris of Sagittarius extends up to 60 degrees from its center, suggesting that the stream of this galaxy completely wraps the Milky Way in an almost polar orbit. Our negative detections also suggest the stream is narrow, supporting a nearly spherical Milky Way dark matter halo potential.