Results from Two Surveys Indicating Massive Lensing Objects in The Halo of M31

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We present results from two separate surveys, one complete and one preliminary, of microlensing of stars in the nearby spiral, M31. This is the only case of a disk galaxy beyond our own which bears on whether halo microlensing objects compose a significant fraction of galaxy dark matter throughout the Universe. The VATT/Columbia survey was conducted primarily in 1997-1999 at the VATT and MDM 1.3-meter telescopes (with extended baseline coverage from 1995), and uncovered 4 candidate microlensing events in M31. A comparison with event rate models indicates that 5 +- 3% of the halo dark matter is composed of roughly stellar masses (assuming a singular, isothermal sphere halo distribution, and likely implying a larger halo fraction for other models). The second survey, MEGA, was conducted at the INT, KPNO 4-meter, MDM 2.4-meter and 1.3-meter telescope in 1999-2002. Here we present 14 candidate microlensing events from the first two years on the INT. While we have not completed a detailed efficiency/rate model calculation, we note that these data are strongly concentrated on the far side of M31's disk, a strong signature for the presence of a microlensing halo. This asymmetry is significant at the 1.5 sigma level. These two independent studies together strongly suggest that a significant fraction of the dark matter halo of M31 is composed of roughly stellar-mass objects. By the time of the meeting we anticipate further results from MEGA which will bear further on whether a large fraction of the dark matter in galaxies is composed of these now-mysterious objects.

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