- L E Strigari, J S Bullock, +4 authors M G Walker
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⊙ = 197.5± 3.8 km s−1 and a velocity dispersion of σo = 14.0± 3.2 km s−1. We use the somewhat large velocity dispersion and the implied highly radial orbit, along with morphological evidence from Grillmair (2009) 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. Subject headings: galaxies: individual (Boötes III dwarf spheroidal) – galaxies: kinematics and dynamics – Galaxy: structure – Local Group 1Dept. of Astronomy, University of Virginia, P. O. Box 400325, Charlottesville, VA 22904-4325, USA (jcarlin, dln5q, email@example.com) 2Spitzer Science Center, 1200 E. California Blvd., Pasadena, CA 91125, USA (firstname.lastname@example.org) 3Yale University, Astronomy Department, P.O. Box 208101, New Haven, CT 06520-8101, USA (email@example.com) 1Observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.