The Search for Dark Matter in Draco and Ursa Minor: A Three Year Progress Report

Abstract

We report the cumulative results of an on-going effort to measure the stellar velocity dispersion in two nearby dwarf spheroidal galaxies. Radial velocities having an accuracy ≲ 2 km s−1 have now been secured for ten stars in Ursa Minor and eleven stars in Draco (including 16 K giants and 5 C types). Most objects have been observed at two or more epochs. Stars having non-variable velocities yield in both dwarfs a large (∼ 10 km s−1) dispersion. These results cannot be explained by atmospheric motions, and circumstantial evidence suggests that the effects of undetected binaries are also not likely to be important. Instead, it seems that both spheroidals contain a substantial dark matter component, which therefore must be “cold” in form.