POINT-AGAPE pixel lensing survey of M 31. Evidence for a MACHO contribution to galactic halos

December 2005 • 2005A&A...443..911C

Authors • Calchi Novati, S. • Paulin-Henriksson, S. • An, J. • Baillon, P. • Belokurov, V. • Carr, B. J. • Crézé, M. • Evans, N. W. • Giraud-Héraud, Y. • Gould, A. • Hewett, P. • Jetzer, Ph. • Kaplan, J. • Kerins, E. • Smartt, S. J. • Stalin, C. S. • Tsapras, Y. • Weston, M. J. • POINT-AGAPE Collaboration

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 M_⊙. This lower bound drops to 8% for MACHOs with masses ~0.01 M_⊙. 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.

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Sebastiano Calchi Novati

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