AAS 203rd Meeting, January 2004
Session 120 Gravitational Lensing
Poster, Thursday, January 8, 2004, 9:20am-4:00pm, Hanover Hall

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

A.P.S. Crotts (Columbia U.), J.T.A. de Jong (Kapteyn A.I.), R.R. Uglesich (Columbia U.), E.A. Baltz (Stanford U.), K.H. Kuijken (Kapteyn A.I.), P.D. Sackett (M.S.S.S.O.), R.P. Boyle (Vatican O.R.G.), W.J. Sutherland (I.o.A.), P. Cseresnjes (Columbia U.), C.J. Corbally (Vatican O.R.G.), G. Gyuk (U. of Chicago), L.M. Widrow (Queen's U.)

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.

The author(s) of this abstract have provided an email address for comments about the abstract: arlin@astro.columbia.edu

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