Exploring Halo Substructure with Giant Stars: Spectroscopy of Stars in the Galactic Anticenter Stellar Structure

To determine the nature of the recently discovered, ringlike stellar structure at the Galactic anticenter, we have collected spectra of a set of presumed constituent M giants selected from the Two Micron All Sky Survey Point Source Catalog. Radial velocities have been obtained for stars spanning ~100°, exhibiting a trend in velocity with Galactic longitude and an estimated dispersion of σ_v=20+/-4 km s^-1. A mean metallicity [Fe/H] = -0.4+/-0.3 measured for these stars combines with previous evidence from the literature to suggest a population with a significant metallicity spread. In addition, a curious alignment of at least four globular clusters of lower mean metallicity is noted to be spatially and kinematically consistent with this stellar distribution. We interpret the M giant sample position and velocity variation with Galactic longitude as suggestive of a satellite galaxy currently undergoing tidal disruption in a noncircular, prograde orbit about the Milky Way.