AGAPE:a microlensing search in the direction of M31

The M31 galaxy in Andromeda is the nearest large galaxy after the Small and Large Magellanic Clouds. It is a giant galaxy, roughly 2 times as large as our Milky Way, and has its own halo. As pointed by some of us [1, 2] and independantly by A. Crotts [3], M31 provides a rich eld of stars to search for MACHO's in galactic halos by gravitational microlensing [4]. M31 is a target complementary to the Magellanic Clouds used by the current experiments [5, 6]. It is complementary in that it allows to probe the halo of our galaxy in a direction very di erent from that of the LMC. Moreover, the fact that M31 has its own halo and is tilted with respect to the line of sight provides a very interesting signature [3] : assuming an approximately spherical halo for M31, the far side of the disk lies behind a larger amount of M31 dark matter, therefore more microlensing events are expected on the far side of the disk. Such an asymmetry could not be faked by variable stars. In other words, M31 seems very appropriate to detect brown dwarfs through microlensing. However, as very few stars of M31 are resolved, we had to develop a new approach to look for microlensing by monitoring the pixels of a CCD, rather than individual stars [1, 2]. The AGAPE collaboration has set out to implement this idea.