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 unresolv ed 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 mo dels predict a general absence of stars in the color-magnitude range in whi ch these objects are found. However, these observations are consistent with recently developed models of old white dwarfs with hydrogen atmospheres, w hose color, contrary to previous expectations, has been shown to be blue. I f these apparently moving objects are indeed old white dwarfs with hydrogen atmospheres and masses near 0.5 M-., they have ages of approximately 12 Gy r and a local mass density that is sufficient, within the large uncertainti es arising from the small size of the sample, to account for the entire mis sing Galactic dynamical mass.