Evidence of Halo Microlensing in M31

We have completed an intensive monitoring program of two fields on either side of the center of M31, and report here on the results concerning microlensing of stars in M31. These results stem from a three-year study (the VATT/Columbia survey) of microlensing and variability of M31 stars, emphasizing microlensing events of 3 day to 2 month timescales and likely due to masses in M31. These observations were conducted intensively from 1997-1999, with baselines 1995-present, at the Vatican Advanced Technology Telescope and the 1.3-meter telescope at MDM Observatory, with additional data from the Isaac Newton Telescope, including about 200 epochs total. The two fields monitored cover 560 square arcminutes total, positioned along the minor axis on either side of M31. Candidate microlensing events are subject to a number of tests discussed here with the purpose of distinguishing microlensing from variable star activity. A total of four probable microlensing events, when compared to carefully computed event rate and efficiency models, indicate a marginally significant microlensing activity above that expected for the stars alone in M31 (and the Galaxy) acting as lenses. A maximum likelihood analysis of the distribution of events in timescale and across the face of M31 indicate a microlensing dark matter halo fraction consistent with that seen in our Galaxy towards the Large Magellanic Cloud (Alcock et al. 2000a). Specifically, for a nearly singular isothermal sphere model, we find a microlensing halo mass fraction fb = 0.29 +0.30 −0.13 of the total dark matter, and a poorly constrained lensing component mass (0.02 to 1.5M⊙, 1 σ limits). This study serves as the prototype for a larger study approaching completion; between the two there is significant evidence for an asymmetry in the distribution of microlensing events across the face of M31, and therefore a large population of halo microlensing dark matter objects. Subject headings: gravitational lensing — galaxies: individual (M31) — galaxies: halos — dark matter