Unbound dwarf spheroidal galaxies and the mass of the Milky Way

There are reasonable dynamical arguments to support the claim that at least some of the Local Group dwarf spheroidal (dSph) galaxies are not internally bound collections of stars. There are now even a few observations that bolster this view. This Letter points out that the lifetimes of these systems may be longer than is commonly appreciated, and that (as unbound systems) dynamical observations of the dSph may tell us much more about the mass and dark matter content of the Milky Way (MW) than they do about the dark matter content of the dwarf spheroidal.