Implications of White Dwarf Galactic Halos

Abstract

Motivated by recent measurements which suggest that roughly half the mass of the galactic halo may be in the form of white dwarfs, we study the implications of such a halo. We first use current limits on the infrared background light and the galactic metallicity to constrain the allowed initial mass function (IMF) of the stellar population that produced the white dwarfs. The IMF must be sharply peaked about a characteristic mass scale $M_C \approx 2.3 M_\odot$. Since only a fraction of the initial mass of a star is incorporated into the remnant white dwarf, we argue that the mass fraction of white dwarfs in the halo is likely to be 25\% or less, and that 50\% is an extreme upper limit. We use the IMF results to place corresponding constraints on the primordial initial conditions for star formation. The initial conditions must be much more homogeneous and skewed toward higher temperatures ($T_{\rm gas} \sim$ 200 K) than the conditions which lead to the present day IMF. Next we determine the luminosity function of white dwarfs. By comparing this result with the observed luminosity function, we find that the age of the halo population must be greater than $\sim 16$ Gyr. Finally, we calculate the radiative signature of a white dwarf halo. This infrared background is very faint, but is potentially detectable with future observations.