A Low‐Latitude Halo Stream around the Milky Way

We present evidence for a ring of stars in the plane of the Milky Way,extending at least from l = 180 deg to l = 227 deg with turnoff magnitude $g\sim 19.5$; the ring could encircle the Galaxy. We infer that the low Galacticlatitude structure is at a fairly constant distance of R = 18 +/- 2 kpc fromthe Galactic Center above the Galactic plane, and has R = 20 +/- 2 kpc in theregion sampled below the Galactic plane. The evidence includes five hundredSDSS spectroscopic radial velocities of stars within 30 degrees of the plane.The velocity dispersion of the stars associated with this structure is found tobe 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) degrees. The structurerotates in the same prograde direction as the Galactic disk stars, but with acircular velocity of 110+/-25 km/s. The narrow measured velocity dispersion isinconsistent with power law spheroid or thick disk populations. We compare thevelocity dispersion in this structure with the velocity dispersion of stars inthe Sagittarius dwarf galaxy tidal stream, for which we measure a velocitydispersion of 20 km/s at (l, b) = (165, -55) deg. We estimate a preliminarymetallicity 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 consistentwith that of the spheroid population. We interpret our measurements as evidencefor a tidally disrupted satellite of $2 \times 10^7$ to $5 \times 10^8 \rmM_\odot$ which rings the Galaxy.Comment: Revised to include 2 page erratum (appended to original paper), 18 pages, Latex, 12 figures (3 revised), for data table: choose 'other formats', download source to find table1rev.tx