Observational Prospects for Extra-Galactic Microlensing Events

We consider the feasibility of directly observing gravitational microlensing in extra-galactic sources, whose stars are not generally resolved. This precludes use of the simple optical depth to microlensing formulation, which is applicable only to resolved stars. We, instead, extend this method to consider observational constraints, such as seeing, sky background and minimum detectable change in magnitude. Our analysis provides quantitative relations between these constraints and the expected observational results, event duration and number of detections per year. We find that an ambitious ground-based observer should detect several events per year in M31. We also consider detection of microlensing in visual binary galaxies. We find that, although these objects may present hundreds of events per year, extremely short durations would yield poor prospects for observation.