Gravitational Microlensing by Globular Clusters

Abstract

Stars in globular clusters can act either as sources for MACHOs (Massive Astrophysical Compact Halo Objects) located along the line of sight or as lenses for more distant background stars. Although the expected rate of microlensing events is small, such observations can lead to very useful results. In particular, one could get information on the shape of the galactic halo along different lines of sight, allowing to better constrain its total dark matter content. Moreover, on can also infer the total dark matter content of globular clusters, which is presently not well known. To this latter purpose, we analyse the microlensing events towards the galactic bulge, which lie close to the three globular clusters NGC 6522, NGC 6528 and NGC 6540. We find evidence that some microlensing events are indeed due to MACHOs located in the globular clusters, suggesting, therefore, that these clusters contain a significant amount of dark matter.