The Ghost of Sagittarius and Lumps in the Halo of the Milky Way

We identify new structures in the halo of the Milky Way Galaxy frompositions, colors and magnitudes of five million stars detected in the SloanDigital Sky Survey. Most of these stars are within 1.26 degrees of thecelestial equator. We present color-magnitude diagrams (CMDs) for stars in twopreviously discovered, tidally disrupted structures. The CMDs and turnoffcolors are consistent with those of the Sagittarius dwarf galaxy, as had beenpredicted. In one direction, we are even able to detect a clump of red stars,similar to that of the Sagittarius dwarf, from stars spread across 110 squaredegrees of sky. Focusing on stars with the colors of F turnoff objects, weidentify at least five additional overdensities of stars. Four of these may bepieces of the same halo structure, which would cover a region of the sky atleast 40 degrees in diameter, at a distance of 11 kpc from the Sun (18 kpc fromthe center of the Galaxy). The turnoff is significantly bluer than that ofthick disk stars, and closer to the Galactic plane than a power-law spheroid.We suggest two models to explain this new structure. One possibility is thatthis new structure could be a new dwarf satellite of the Milky Way, hidden inthe Galactic plane, and in the process of being tidally disrupted. The otherpossibility is that it could be part of a disk-like distribution of stars whichis metal-poor, with a scale height of approximately 2 kpc and a scale length ofapproximately 10 kpc. The fifth overdensity, which is 20 kpc away, is somedistance from the Sagittarius dwarf streamer orbit and is not associated withany known structure in the Galactic plane. It is likely that there are manysmaller streams of stars in the Galactic halo.Comment: ApJ, in press; 26 figures including several in colo