Exploring Halo Substructure with Giant Stars: The Dynamics and Metallicity of the Dwarf Spheroidal in Boötes

We report the results of a spectroscopic study of the Boötes (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 σ₀=6.6+/-2.3 km s^-1. This implies a mass on the order of 1×10⁷ M_solar, similar to the inferred masses of other Galactic dSphs. Adopting a total Boo luminosity of L=1.8×10⁴ to 8.6×10⁴ L_solar implies M/L~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] ~-2.5, which would make it the most metal-poor dSph known to date.