From observations, little is known about the nature and structure of galaxy halos. Stellar streams have been found around the Galaxy and our nearest neighbour, M31, presumably remnants of past accretion events. These represent only a very small fraction of the total mass of the halos. From microlensing surveys towards the Magellanic Clouds there are indications that part of the Galactic halo might consist of dark compact objects, but the exact amount is uncertain. M31 provides a significantly

more »

... tter target for a microlensing survey than the Magellanic Clouds. Due to the lensing geometry and the availability of lines-of-sight through very dense parts of the M31 halo, the microlensing rates are greatly enhanced. The Microlensing Exploration of the Galaxy and Andromeda (MEGA) is performing a large-scale microlensing survey towards M31 using several telescopes. We present the first candidate microlensing events from the INT data (de Jong et al. 2004 ). These results are preliminary, but suggestive of the presence of a microlensing halo. Figure 1 : M31's disk has in inclination of 77 degrees to our line-of-sight. Because of this, the path length through the dark halo is much lnger towards the far side of the disk, greatly increasing the microlensing probability with respect to the near side. Therefore, the microlensing signature of the halo will be much more asymmetric than that of microlensing by stars in M31.