MEGACAM offers the CFHT community the opportunity to impact on a number of areas in astronomy where large samples are important. One such area is the stellar component of the Galactic halo, and, in particular, the halo white dwarfs. The number and luminosity function of these objects are of interest for two problems of fundamental importance: the Galactic dark matter and the age of the Universe.
Recent microlensing results have indicated that mean MACHO masses are near 0.5 Msun. This mass is typical of that of white dwarfs. The lensing rate is such that upwards of 40% of the dark matter in the Galaxy could be contained in MACHOs. If MACHOs are white dwarfs they must be old objects, formed early-on in the life of the Galaxy. These objects will then have low luminosities and be extremely cool. If we take their age as 13 Gyrs, current white dwarf cooling models predict that their effective temperatures will be in the range of 3200K, their absolute I magnitudes near 16 and their (V-I) colours about 1.6. What will an imaging survey covering 25 square degrees and descending to a limiting I magnitude of 24 reveal about the white dwarf population in the Galactic halo? The number of MACHO white dwarfs in such a survey will be about 630 which is roughly a factor of 20 larger than the expected number of disk white dwarfs. Such a survey can then clearly differentiate between the white dwarf-MACHO model for the dark matter and other MACHO or non-MACHO models. In such an imaging survey it will be necessary to revisit each field a number of times in order that the proper motions of the stars be measured. This will be an important way of distinguishing between white dwarfs and low luminosity main sequence stars and of separating stars from compact galaxies. Old white dwarfs at the limiting magnitude of the survey will have typical proper motions near 0.07"/year which is easily detected.
The age of the Universe remains controversial. In standard inflationary cosmology, the expansion age, using H0 = 70 km/sec/Mpc, is a relatively young 9.5 Gyr. By contrast, the absolute ages of the globular clusters implies a Universe with a lower limit to its age nearer to 14 Gyr. Some recent age determinations using a distance scale derived from Hipparcos parallaxes of local metal-poor subdwarfs have suggested an age reduction to around 11 Gyr. These results are also controversial in that they produce serious problems with the cluster white dwarfs, the distance to M31 globular clusters and the current generation of stellar models. A difference of about 4 Gyr between the expansion age and the globular cluster ages can potentially be resolved by allowing for a cosmological constant, but such a solution will not be compelling until all avenues to check population II age estimates are exhausted. The halo white dwarfs provide just such an opportunity. Isolating a sample of halo white dwarfs will allow for construction of the halo white dwarf luminosity function. The turn-over in this function at low luminosity will provide a time since the white dwarfs began to cool which is a strong lower limit to the age of the Universe. Exactly this same technique was used for disk white dwarfs to derive an age for the Galactic disk near 10 Gyr. This project, which is ideal for MEGACAM, is independent of whether or not white dwarfs are associated with the Galactic dark matter.