High-velocity white dwarfs: Thick disk, not dark matter

We present an alternative interpretation of the nature of the extremely coo l, high-velocity white dwarfs identified by Oppenheimer et al. in a high-la titude astrometric survey. We argue that the velocities of the majority of the sample are more consistent with the high-velocity tail of a rotating po pulation, probably the thick disk, rather than with a pressure-supported ha lo system. Indeed, the observed numbers are well matched by predictions bas ed on the kinematics of a complete sample of nearby M dwarfs. Analyzing onl y stars showing retrograde motion gives a local density close to that expec ted for white dwarfs in the stellar (R-3.5) halo. Under our interpretation, none of the white dwarfs need be assigned to the dark matter heavy halo. H owever, luminosity functions derived from observations of these stars can s et important constraints on the age of the oldest stars in the Galactic dis k.