M31: A Unique Laboratory for Gravitational Microlensing

A search for microlensing of stars in the disk M31 by massive, condensed halo objects (MACHOs) within both M31 and the Galaxy offers interesting advantages over attempts to detect such objects by observing nearby stars (such as in the Galactic bulge or the Magellanic Clouds). Due to the smaller angular size of stars in M31, such a search is sensitive to a much lower mass limit (~3 x 10^-10^ M_sun_) than searches in nearer fields (~10^-6^ M_sun_) for MACHOs in the Galactic halo. Due to the greater mass of M31 and the favorable geometry of its nearly edge-on disk, many more MACHO lensing events might also be expected due to objects in M31's halo. This leasing rate may be up to 15 times higher than Galactic halo lensing in any of these experiments. Unlike Galactic MACHO events, however, M31 microlensing can be measured with respect to a control experiment (a field of identical nature except for the expectation of a much reduced microlensing rate). In this way, microlensing due to MACHOs in the halo of M31 can be more easily distinguished from normal variable stars, a difficult background of events likely to plague other searches. We discuss how the observational difficulties of observing a sufficient number of stars in M31 might be overcome.