We report the results of a spectroscopic study of the Boötes III ( BooIII ) stellar overdensity carried out with the Hectospec multifiber spectrograph on the MMT telescope .
Radial velocities have been measured for 193 BooIII candidate stars selected to have magnitudes and colors consistent with its upper main sequence and lower red giant branch , as well as a number of horizontal branch candidates .
From 20 identified candidate BooIII members , we measure a systemic velocity of V _ { \sun } = 197.5 \pm 3.8 km s ^ { -1 } and a velocity dispersion of \sigma _ { o } = 14.0 \pm 3.2 km s ^ { -1 } .
We use the somewhat large velocity dispersion and the implied highly radial orbit , along with morphological evidence from Grillmair ( 13 ) and stellar abundances , to argue that BooIII is likely the first known object observed in a transitional state between being a bound dwarf galaxy and a completely unbound tidal stream .