Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds *,**
CEA, DSM, DAPNIA, Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
2 Laboratoire de l'Accélérateur Linéaire, IN2P3 CNRS, Université de Paris-Sud, 91405 Orsay Cedex, France
3 The Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
4 Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98 bis boulevard Arago, 75014 Paris, France
5 Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique (UMR 5572), 14 av. E. Belin, 31400 Toulouse, France
6 European Southern Observatory (ESO), Casilla 19001, Santiago 19, Chile
7 Department of Astronomy, Ohio State University, Columbus, OH 43210, USA
8 Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 04, France
Accepted: 10 April 2007
Aims. The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called “machos”) could be a major component of the dark matter halo of the Milky Way galaxy. To this end, EROS-2 monitored over 6.7 years stars in the Magellanic clouds for microlensing events caused by such objects.
Methods. In this work, we use only a subsample of bright stars spread over of the LMC and of the SMC. The strategy of using only bright stars helps to discriminate against background events due to variable stars and allows a simple determination of the effects of source confusion (blending). The use of a large solid angle makes the survey relatively insensitive to effects that could make the optical depth strongly direction dependent.
Results. Using this sample of bright stars, only one candidate event was found, whereas ~39 events would have been expected if the Halo were entirely populated by objects of mass . Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud (LMC) due to such lenses is (95% CL), corresponding to a fraction of the halo mass of less than 8%. This optical depth is considerably less than that measured by the MACHO collaboration in the central region of the LMC. More generally, machos in the mass range are ruled out as the primary occupants of the Milky Way Halo.
Key words: Galaxy: halo / cosmology: dark matter / gravitational lensing
© ESO, 2007