Evidence for Halo Microlensing in M31

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

We present results from the VATT/Columbia survey of microlensing and variability of stars in M31, with particular emphasis on events from 3 days to two month timescales that are potentially caused by microlensing by masses in M31. These observations were conducted intensively from 1997-1999, with less intensive monitoring from 1995-present, at the Vatican Advanced Technology Telescope and the 1.3-meter telescope at MDM Observatory, including about 200 epochs total. The fields monitored cover about 560 square arcminutes total, positioned along the minor axis on either side of M31. A total of 4 bona fide and 14 probable microlensing events, when compared to carefully computed event rate and efficiency models, indicate a significant amount of microlensing activity above that expected for the stars alone in M31 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. 2000, ApJ, 542, 281), but significantly different from the no-halo expectation. The masses of these lensing objects are also consistent with the Alcock et al. result, and indicate that stellar-mass halo objects are important in spiral galaxies beyond the LMC sightline through our Galaxy's halo.

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