Microlensing Surveys of M31 in the Wide‐Field Imaging Era

The Andromeda Galaxy (M31) is the closest large galaxy to the Milky Way, thusit is an important laboratory for studying massive dark objects in galactichalos (MACHOs) by gravitational microlensing. Such studies strongly complementthe studies of the Milky Way halo using the the Large and Small MagellanicClouds. We consider the possibilities for microlensing surveys of M31 using thenext generation of wide field imaging telescopes with fields of view in thesquare degree range. We consider proposals for such imagers both on the groundand in space. For concreteness, we specialize to the SNAP proposal for a spacetelescope and the LSST proposal for a ground based telescope. We find that amodest space-based survey of 50 visits of one hour each is considerably betterthan current ground based surveys covering 5 years. Crucially, systematiceffects can be considerably better controlled with a space telescope because ofboth the infrared sensitivity and the angular resolution. To be competitive, 8meter class wide-field ground based imagers must take exposures of severalhundred seconds with several day cadence.Comment: 10 pages, 4 figures, 2 table