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; the ring could encircle the Galaxy. 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 five hundred SDSS spectroscopic radial velocities of stars within 30 deg 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) degrees. The structure co-rotates with the Galactic disk stars at 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) degrees. We interpret our measurements as evidence for a tidally disrupted satellite of 2 \ 10⁷ to 5 \ 10⁸ solar masses which rings the Galaxy.