Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds

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 millions of stars in the Magellanic clouds for microlensing events caused by such objects. About $33\times10^6$ Magellanic stars were observed over a period of 6.7 years. Unlike all previous studies of Magellanic microlensing, we use only a subsample of $7\times10^6$ 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 $\sim42$ events would have been expected if the Halo were entirely populated by objects of mass $M\sim0.4M_{\odot}$. Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud (\object{LMC}) due to such lenses is $\tau<0.36\times10^{-7}$ (95%CL), corresponding to a fraction of the halo mass of less than 7%. This result is at variance with the measurement of the MACHO collaboration. More generally, machos in the mass range $10^{-7}M_\odot<M<5M_{\odot}$ are ruled out as the primary occupants of the Milky Way Halo.