Limits on the Halo White Dwarf Component of Baryonic Dark Matter from the Hubble Deep Field

The MACHO collaboration lensing event statistics suggest that a significantfraction of the dark galactic halo can be comprised of baryonic matter in theform of white dwarf stars with masses between 0.1 and 1.0 \Msun . Such a halowhite dwarf population, in order to have escaped detection by those who observethe white dwarf luminosity function of the disk, must have formed from an oldpopulation. The observations indicate that the number of halo white dwarfs percubic parsec per unit bolometric magnitude is less than $10^{-5}$ at$10^{-4.5}$\Lsun; the number must rise significantly at lower luminosities toprovide the needed baryonic halo mass. Such white dwarfs may easily escapedetection in most current and earlier surveys. Though it is limited in angularextent, the {\em Hubble Deep Field} (HDF) probes a sufficient volume of thegalactic halo to provide interesting limits on the number of halo white dwarfstars, and on the fraction of the halo mass that they can make up. If the HDFfield can be probed for stars down to $V=29.8$ then the MACHO result suggeststhat there could be up to 12 faint halo white dwarfs visible in the HDF.Finding (or not finding) these stars in turn places interesting constraints onstar formation immediately following the formation of the galaxy.Comment: 10 pages, AASTEX, 1 table, no figures, accepted for publication in Ap.J. Letter