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The origin of ultra-compact dwarfs (UCDs)--objects larger and more massivethan typical globular clusters (GCs), but more compact than typical dwarfgalaxies--has been hotly debated in the 15 years since their discovery. Evenwhether UCDs should be considered galactic in origin, or simply the mostextreme GCs, is not yet settled. We present the dynamical properties of 97spectroscopically confirmed UCDs (rh >~10 pc) and 911 GCs associated withcentral cD galaxy of the Virgo cluster, M87. Our UCDs, of which 89% have M_star> ~2X10^6 M_sun and 92% are as blue as the classic blue GCs, nearly triple thesample of previous confirmed Virgo UCDs, providing by far the best opportunityfor studying the global dynamics of a UCD system. We found that (1) UCDs have asurface number density profile that is shallower than that of the blue GCs inthe inner ~ 70 kpc and as steep as that of the red GCs at larger radii; (2)UCDs exhibit a significantly stronger rotation than the GCs, and the blue GCsseem to have a velocity field that is more consistent with that of thesurrounding dwarf ellipticals than with that of UCDs; (3) UCDs have a radiallyincreasing orbital anisotropy profile, and are tangentially-biased at radii < ~40 kpc and radially-biased further out. In contrast, the blue GCs become moretangentially-biased at larger radii beyond ~ 40 kpc; (4) GCs with M_star >2X10^6 M_sun have rotational properties indistinguishable from the less massiveones, suggesting that it is the size, instead of mass, that differentiates UCDsfrom GCs as kinematically distinct populations. We conclude that most UCDs inM87 are not consistent with being merely the most luminous and extendedexamples of otherwise normal GCs. The radially-biased orbital structure of UCDsat large radii is in general agreement with the "tidally threshed dwarf galaxy"scenario.

The Next Generation Virgo Cluster Survey. VI. The Kinematics of Ultra-compact Dwarfs and Globular Clusters in M87