Exploring Halo Substructure with Giant Stars. III. First Results from the Grid Giant Star Survey and Discovery of a Possible Nearby Sagittarius Tidal Structure in Virgo

We describe first results of a spectroscopic probe of selected fields fromthe Grid Giant Star Survey. Multifiber spectroscopy of several hundred stars ina strip of eleven fields along delta approximately -17^{circ}, in the range 12<~ alpha <~ 17 hours, reveals a group of 8 giants that have kinematicalcharacteristics differing from the main field population, but that as a groupmaintain coherent, smoothly varying distances and radial velocities withposition across the fields. Moreover, these stars have roughly the sameabundance, according to their MgH+Mgb absorption line strengths. Photometricparallaxes place these stars in a semi-loop structure, arcing in a contiguousdistribution between 5.7 and 7.9 kpc from the Galactic center. The spatial,kinematical, and abundance coherence of these stars suggests that they are partof a diffuse stream of tidal debris, and one roughly consistent with a wrapped,leading tidal arm of the Sagittarius dwarf spheroidal galaxy.Comment: 8 pages including 4 figures. Accepted for publication in ApJ