Dark baryons not in ancient halo white dwarfs

Abstract

Having ruled out the possibility that stellar objects are the main contributor of the dark matter embedding galaxies, microlensing experiments cannot exclude the hypothesis that a significant fraction of the MilkyWay dark halo might be made of MACHOs with masses in the range 0.5−0.8 M๏. Ancient white dwarfs are generally considered the most plausible candidates for such MACHOs. We report the results of a search for such white dwarfs in a proper motion survey covering a 0.16 sq. deg. field at three epochs at high galactic latitude, and 0.938 sq. deg. at two epochs at intermediate galactic latitude (VIRMOS survey), using the CFH telescope. Both surveys are complete to I = 23, with detection efficiency fading to 0 at I = 24.2. Proper motion data are suitable to separate unambiguously halo white dwarfs identified as belonging to a non rotating system. No candidates were found within the colour−magnitude-proper motion volume where such objects can be safely discriminated from any standard population as well as from possible artefacts. In the same volume, we estimate the maximum white dwarf halo fraction compatible with this observation at different significance levels if the halo is at least 14 gigayears old and under different ad hoc initial mass functions. Our data alone rule out a halo fraction greater than 14% at a 95% confidence level. Combined with two previous investigations exploring comparable volumes, this pushes the limit below 4% (95% confidence level) or below 1% (64% confidence), and implies that if baryonic dark matter is present in galaxy halos, it is not, or is only marginally in the form of faint hydrogen white dwarfs.