POINT-AGAPE pixel lensing survey of M 31

Evidence for a MACHO contribution to galactic halos

¹ Institute for Theoretical Physics, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
² APC, 11 Place Marcelin Berthelot, 75231 Paris, France
³ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁴ CERN, 1211 Genève, Switzerland
⁵ Astronomy Unit, School of Mathematical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
⁶ Université Bretagne-Sud, campus de Tohannic, BP 573, 56017 Vannes Cedex, France
⁷ Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
⁸ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
⁹ Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, UK

Received: 26 March 2005
Accepted: 8 August 2005

Abstract

The POINT-AGAPE collaboration is carrying out a search for gravitational microlensing toward M 31 to reveal galactic dark matter in the form of MACHOs (Massive Astrophysical Compact Halo Objects) in the halos of the Milky Way and M 31. A high-threshold analysis of 3 years of data yields 6 bright, short-duration microlensing events, which are confronted to a simulation of the observations and the analysis. The observed signal is much larger than expected from self lensing alone and we conclude, at the 95% confidence level, that at least 20% of the halo mass in the direction of M 31 must be in the form of MACHOs if their average mass lies in the range 0.5–1 . This lower bound drops to 8% for MACHOs with masses ~0.01 . In addition, we discuss a likely binary microlensing candidate with caustic crossing. Its location, some 32' away from the centre of M 31, supports our conclusion that we are detecting a MACHO signal in the direction of M 31.