High-Velocity White Dwarfs and Galactic Structure

A number of recent investigations suggest that cool white dwarfs are more numerous than predicted by conventional Galactic models and that those stars make a significant contribution to the mass budget of the Milky Way. In particular, there is speculation that cool white dwarfs are linked with the dark-matter halos. This review examines those recent results and matches them against current understanding of the properties of the stellar populations that make up the Milky Way, taking due account of the relative star formation histories of the disk and the thick disk. The new white dwarf observations do not require any additions to the conventional stellar populations of the Milky Way. There is no credible evidence of either an underlying population of ancient white dwarfs or a link between high-velocity degenerates and dark matter. Nonetheless, placed in the proper context, these high-velocity white dwarfs provide interesting insight on the likely history of the Milky Way. We show that the thick disk is likely to contribute ∼20% of Solar Neighborhood white dwarfs. If the thick disk is an old, single-burst population, as favored by most investigations, then those white dwarfs dominate at faint absolute magnitudes. As a result, analyses of the low-luminosity cutoff in the local white dwarf–luminosity function actually set limits on the age of the thick disk, rather than the thin disk. In absolute terms, these results imply that the thick disk is no older than 10 Gyrs, whereas major star formation in the thin disk may not have started until 7 to 8 Gyrs ago. Moreover, the enhanced [α/Fe] ratios in thick-disk and halo main-sequence stars suggest that those two populations have similar ages, implying a relatively young age for the field halo.