High‐Velocity White Dwarfs: Thick Disk, Not Dark Matter

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Abstract
We present an alternative interpretation of the nature of the extremely cool, high-velocity white dwarfs identified by Oppenheimer et al. in a high-latitude astrometric survey. We argue that the velocities of the majority of the sample are more consistent with the high-velocity tail of a rotating population, probably the thick disk, rather than with a pressure-supported halo system. Indeed, the observed numbers are well matched by predictions based on the kinematics of a complete sample of nearby M dwarfs. Analyzing only stars showing retrograde motion gives a local density close to that expected 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. However, luminosity functions derived from observations of these stars can set important constraints on the age of the oldest stars in the Galactic disk.