A near-zero velocity dispersion stellar component in the Canes Venatici dwarf spheroidal galaxy

We present a spectroscopic survey of the newly discovered Canes Venatici dwarf galaxy using the Keck/DEIMOS spectrograph. Two stellar populations of distinct kinematics are found to be present in this galaxy: an extended, metal-poor component, of half-light radius 7.8(-2.1)(+2.4) arcmin, which has a velocity dispersion of 13.9(-2.5)(+3.2) km s(-1), and a more concentrated (half-light radius 3.6(-0.8)(+1.1) arcmin) metal-rich component of extremely low velocity dispersion. At 99 per cent confidence, the upper limit to the central velocity dispersion of the metal-rich population is 1.9 km s(-1). This is the lowest velocity dispersion ever measured in a galaxy. We perform a Jeans analysis on the two components, and find that the dynamics of the structures can only be consistent if we adopt extreme (and unlikely) values for the scalelength and velocity dispersion of the metal-poor population. With a larger radial velocity sample and improved measurements of the density profile of the two populations, we anticipate that it will be possible to place strong constraints on the central distribution of the dark matter in this galaxy.