MACHOs in M 31? Absence of evidence but not evidence of absence

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands e-mail: jdejong@astro.rug.nl
² Department of Physics, Engineering Physics, and Astronomy, Queen's University, Kingston, ON K7L 3N6, Canada
³ Columbia Astrophysics Laboratory, 550 W 120th St., Mail Code 5247, New York, NY 10027, USA
⁴ Sterrewacht Leiden, University of Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁵ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, PO Box 20450, MS 29, Stanford, CA 94309, USA
⁶ Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
⁷ Research School of Astronomy and Astrophysics, Australian National University, Mt. Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia
⁸ Laboratory of Applied Mathematics, Box 1012, Mount Sinai School of Medicine, One Gustave L. Levy Place, New York, NY 10029, USA
⁹ Institute of Astronomy, Madingley Rd, Cambridge CB3 0HA, UK

Received: 11 July 2005
Accepted: 27 September 2005

Abstract

We present results of a microlensing survey toward the Andromeda Galaxy (M 31) carried out during four observing seasons at the Isaac Newton Telescope (INT). This survey is part of the larger microlensing survey toward M 31 performed by the Microlensing Exploration of the Galaxy and Andromeda (MEGA) collaboration. Using a fully automated search algorithm, we identify 14 candidate microlensing events, three of which are reported here for the first time. Observations obtained at the Mayall telescope are combined with the INT data to produce composite lightcurves for these candidates. The results from the survey are compared with theoretical predictions for the number and distribution of events. These predictions are based on a Monte Carlo calculation of the detection efficiency and disk-bulge-halo models for M 31. The models provide the full phase-space distribution functions (DFs) for the lens and source populations and are motivated by dynamical and observational considerations. They include differential extinction and span a wide range of parameter space characterised primarily by the mass-to-light ratios for the disk and bulge. For most models, the observed event rate is consistent with the rate predicted for self-lensing – a MACHO halo fraction of 30% or higher can be ruled at the 95% confidence level. The event distribution does show a large near-far asymmetry hinting at a halo contribution to the microlensing signal. Two candidate events are located at particularly large projected radii on the far side of the disk. These events are difficult to explain by self lensing and only somewhat easier to explain by MACHO lensing. A possibility is that one of these is due to a lens in a giant stellar stream.