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

摘要

We report the results of a spectroscopic study of the Bootes (Boo) dwarfspheroidal (dSph) galaxy carried out with the WIYN telescope and the Hydramultifiber spectrograph. Radial velocities have been measured for 58 Boocandidate stars selected to have magnitudes and colors consistent with its redand asymptotic giant branches. Within the 13' half-light radius, seven membersof Boo yield a systemic velocity of V_r=95.6+-3.4 km/s and a velocitydispersion of 6.6+-2.3 km/s. This implies a mass on the order of 1 x 10^7M_sun, similar to the inferred masses of other Galactic dSphs. Adopting a totalBoo luminosity of L=1.8 x 10^4 L_sun to 8.6 x 10^4 L_sun implies M/L ~ 610 to130, making Boo, the most distorted known Milky Way dwarf galaxy, potentiallyalso the darkest. From the spectra of Boo member stars we estimate itsmetallicity to be [Fe/H] ~ -2.5, which would make it the most metal poor dSphknown to date.