Exploring halo substructure with giant stars:: The dynamics and metallicity of the dwarf spheroidal in Bootes

We report the results of a spectroscopic study of the Bootes (Boo) dwarf spheroidal (dSph) galaxy carried out with the WIYN telescope and the Hydra multifiber spectrograph. Radial velocities have been measured for 58 Boo candidate stars selected to have magnitudes and colors consistent with its red and asymptotic giant branches. Within the 13' half-light radius, seven members of Boo yield a systemic velocity of V-r = 95.6 +/- 3.4 km s(-1) and a velocity dispersion of km sigma(0) = 6.6 +/- 2.3 km s(-1). This implies a mass on the order of 1 x 10(7) M-circle dot, similar to the inferred masses of other Galactic dSphs. Adopting a total Boo luminosity of L = 1.8 x 10(4) to 8.6 x 10(4) L-circle dot implies M/L similar to 610 - 130, making Boo, the most distorted known Milky Way dwarf galaxy, potentially also the darkest. From the spectra of Boo member stars we estimate its metallicity to be [Fe/H], which would similar to 2.5 make it the most metal-poor dSph known to date.