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

To determine the nature of the recently discovered, ring-like stellarstructure at the Galactic anticenter, we have collected spectra of a set ofpresumed constituent M giants selected from the 2MASS point source catalog.Radial velocities have been obtained for stars spanning ~100 degrees,exhibiting a trend in velocity with Galactic longitude and an estimateddispersion of 20 +/- 4 km/sec. A mean metallicity [Fe/H] = -0.4 +/- 0.3measured for these stars combines with previous evidence from the literature tosuggest a population with a significant metallicity spread. In addition, acurious alignment of at least four globular clusters of lower mean metallicityis noted to be spatially and kinematically consistent with this stellardistribution. We interpret the M giant sample position and velocity variationwith Galactic longitude as suggestive of a satellite galaxy currentlyundergoing tidal disruption in a non-circular, prograde orbit about the MilkyWay.Comment: (1) University of Virginia, 4 pages, 3 figures, accepted for publication in The Astrophysical Journal Letter