ABSOLUTE PROPER MOTIONS TO B-SIMILAR-TO-22.5 - EVIDENCE FOR KINEMATICAL SUBSTRUCTURE IN HALO FIELD STARS

Radial velocities have been obtained for six of nine stars identified on the basis of similar distances and common, extreme transverse velocities in the proper motion survey of Majewski (1992) as a candidate halo moving group at the north Galactic pole. These radial velocities correspond to velocities perpendicular to the Galactic plane which span the range -48 +/- 21 to -128 +/- 9 km s-1 (but a smaller range, - 48 +/- 21 to -86 +/- 19 km s-1, when only our own measurements are considered), significantly different than the expected distribution, with mean 0 km s-1, for a random sample of either halo or thick disk stars. The probability of picking such a set of radial velocities at random is less than 1%. Thus the radial velocity data support the hypothesis that these stars constitute part of a halo moving group or star stream at a distance of approximately 4-5 kpc above the Galactic plane. If real, this moving group is evidence for halo phase space substructure which may be the fossil remains of a destroyed globular cluster, Galactic satellite, or Searle & Zinn (1978) ''fragment.''