THE WHITE-DWARF LUMINOSITY FUNCTION - A POSSIBLE PROBE OF THE GALACTIC HALO

The dynamically inferred dark halo mass density, amounting to ∼ 0.01 M⊙ pc-3 at the Sun's Galactocentric radius, can be composed of faint white dwarfs provided that the halo formed in a sufficiently early burst of star formation. The model is constrained by the observed disk white dwarf luminosity function which falls off below log (L/L⊙) = -4.4 due to the onset of star formation in the disk. By using a narrow range for the initial mass function (2-6 M⊙) and an exponentially decaying halo star formation rate with an e-folding time equal to the free-fall time (108 yr), we can allow all the halo dark matter to be in cool white dwarfs which lie beyond the falloff in the disk luminosity function. Such halos can be no less than 12 Gyr old and require an enormous but brief initial star formation rate some 800 times the constant star formation rate assumed for the disk. Although it is unlikely that all the dark matter is in these dim white dwarfs, we predict a definite signature in the low-luminosity end of the white dwarf luminosity function even if they comprise only 1% of the dark matter. Current CCD surveys should answer the question of the existence of this population within the next few years.