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5.4. Implications for microlensing

Microlensing surveys are concentrating on the Galactic bulge, the Galactic halo through monitoring of sight lines toward the Magellanic Clouds, and on M31 through pixel lensing. All of these surveys concentrate on fields with high-density, luminous background populations. Results and constraints on dark matter from these surveys are discussed elsewhere in this volume.

The remaining Local Group dwarf galaxies are less well suited for classical microlensing studies. Advantages of using other dwarfs are that one can probe additional lines of sight and can take advantage of the large optical depth to microlensing since the sources are outside of the Milky Way halo. Also, in nearby dSphs crowding won't be much of a problem. However, the efficiency of such studies would be drastically reduced as compared to the ongoing studies since the targets are faint and stellar densities are low. This requires not only longer exposure times or larger telescopes but also implies much longer time scales before a significant number of events can be observed.

As discussed earlier the high velocity dispersions in low-mass dwarfs may arise from large amounts of dark matter. This increases the possibility that one may observe self-lensing when monitoring dwarfs rather than events in the Galactic halo, an effect that is negligible when turning to distant Galactic globular clusters instead (Gyuk & Holder 1998). As always, variable stars may act as contaminants. Future large survey telescopes such as the proposed Dark Matter Telescope (Tyson, Wittman, & Angel 2000) can provide routinely deep exposures of nearby Local Group dwarf galaxies once per night as a regular by-product of their search for cosmological weak lensing.