Limits on the Machos from EROS-2

Abstract

We present the results of the EROS-2 search for microlensing of stars in the Magellanic Clouds. Such events could be caused by macho dark matter in the Milky Way halo. Unlike all previous studies of Magellanic microlensing, we use only a subsample of 7 × 10 6 bright stars. This strategy minimizes backgrounds due to variable stars and ensures accurate determination of lensing parameters by minimizing source confusion (blending). Using this sample of bright stars, only one candidate event was found, whereas ∼42 events would have been expected if the Halo were entirely populated by objects of mass M ∼ 0.4 M ⊙ . Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud (LMC) due to such lenses is τ 0.36 × 10 −7 (95%CL), corresponding to a fraction of the halo mass of less than 7%. This limit on the optical depth is at variance with the measurement of the MACHO collaboration if the microlensing rate is independent of position in the LMC, as expected for halo lenses.