We present evidence for a ring of stars in the plane of the Milky Way , extending at least from l = 180 ^ { \circ } to l = 227 ^ { \circ } with turnoff magnitude g \sim 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 \pm 2 kpc from the Galactic Center above the Galactic plane , and has R = 20 \pm 2 kpc in the region sampled below the Galactic plane .
The evidence includes five hundred SDSS spectroscopic radial velocities of stars within 30 ^ { \circ } of the plane .
The velocity dispersion of the stars associated with this structure is found to be 27 \ > km\ > s ^ { -1 } at ( l,b ) = ( 198 ^ { \circ } , -27 ^ { \circ } ) , 22 km\ > s ^ { -1 } at ( l,b ) = ( 225 ^ { \circ } , 28 ^ { \circ } ) , 30 km\ > s ^ { -1 } at ( l,b ) = ( 188 ^ { \circ } , 24 ^ { \circ } ) , and 30 km\ > s ^ { -1 } at ( l,b ) = ( 182 ^ { \circ } , 27 ^ { \circ } ) .
The structure rotates in the same prograde direction as the Galactic disk stars , but with a circular velocity of 110 \pm 25 \ > km\ > s ^ { -1 } .
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 ^ { -1 } at ( l,b ) = ( 165 ^ { \circ } , -55 ^ { \circ } ) .
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 2 \times 10 ^ { 7 } to 5 \times 10 ^ { 8 } M _ { \odot } which rings the Galaxy .