Faint, moving objects in the Hubble Deep Field: Components of the dark halo?

The deepest optical image of the sky, the Hubble Deep Field, obtained with the Hubble Space Telescope in 1995 December, has been compared to a similar image taken in 1997 December. Two very faint, blue, isolated, and unresolved objects are found to display a substantial apparent proper motion, 23 +/- 5 and 26 +/- 5 mas yr(-1); a further three objects at the detection limit of the second-epoch observations may also be moving. Galactic structure models predict a general absence of stars in the color-magnitude range in which these objects are found. However, these observations are consistent with recently developed models of old white dwarfs with hydrogen atmospheres, whose color, contrary to previous expectations, has been shown to be blue. If these apparently moving objects are indeed old white dwarfs with hydrogen atmospheres and masses near 0.5 M-., they have ages of approximately 12 Gyr and a local mass density that is sufficient, within the large uncertainties arising from the small size of the sample, to account for the entire missing Galactic dynamical mass.