Microlensing in M31 - The MEGA Survey's Prospects and Initial Results

Abstract

January 2000 completes the first season of intensive, wide-field observations of microlensing and stellar variability in M31 by MEGA (Microlensing Exploration of the Galaxy and Andromeda) at the Isaac Newton 2.5m Telescope, the KPNO 4m, and the 1.3m and 2.5m telescopes of MDM Observatory. In preliminary analysis, we detect ~50000 variable objects, including some consistent with microlensing events. We present the level of sensitivity to be reached in our planned three-year program to test for the presence of a significant halo microlensing population in M31, as well as its spatial distribution and mass-function. We also discuss our application of image subtraction to these wide fields and HST WFPC2 Snapshot followup observations to confirm candidates identified from previous years' surveys. We present intermediate results from our smaller-field survey, on the MDM 1.3m and Vatican Advanced Technology 1.8m Telescope, from 1994-1998, wherein we have discovered 8 additional probable microlensing events, over about one-half the time base of the project, in addition to confirming three of our original 6 microlensing candidates from 1995.