2003ApJ...588..824Y

We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l=180° to 227° with turnoff magnitude g∼19.5; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R=18±2 kpc from the Galactic center above the Galactic plane and has R=20±2 kpc in the region sampled below the Galactic plane. The evidence includes 500 Sloan Digital Sky Survey spectroscopic radial velocities of stars within 30° of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km/s at (l,b)=(198°,-27°), 22 km/s at (l,b)=(225°,28°), 30 km/s at (l,b)=(188°,24°), and 30 km/s at (l,b)=(182°,27°). The structure rotates in the same prograde direction as the Galactic disk stars but with a circular velocity of 110±25 km/s. The narrow measured velocity dispersion is inconsistent with power-law spheroid or thick-disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km/s at (l,b)=(165°,-55°). We estimate a preliminary metallicity from the Ca II (K) line and color of the turnoff stars of [Fe/H]=-1.6 with a dispersion of 0.3 dex and note that the turnoff color is consistent with that of the spheroid population. We interpret our measurements as evidence for a tidally disrupted satellite of 2x10⁷ to 5x10⁸ M_☉that rings the Galaxy.