How to find MACHOs in the Virgo Cluster

We discuss the feasibility of finding extra-galactic MACHOs by monitoring quasars behind the Virgo cluster of galaxies. We show that with only a modest observing programme one could detect several MACHOs in the mass range 1 \Theta 10 \Gamma5 \Gamma 2 \Theta 10 \Gamma2 M fi if they make a significant contribution to the mass of Virgo. The contamination by events from cosmologically distributed MACHOs is estimated and is negligible if either the MACHO mass is ¸ ? 10 \Gamma4 M fi or the quasar radius is ¸ ? 3 \Theta 10 15 cm. Key words: dark matter --- galaxies: clusters: general --- gravitational lensing --- quasars: general PACS: 95.30.S,95.35 1 Introduction The optical depth to microlensing toward the LMC is a few \Theta10 \Gamma7 and the detection of halo microlensing events requires extensive monitoring campaigns. The MACHO project, for example, after monitoring several million stars over a 2:3 year period reported only 8 events towards the LMC. In this article we dis..