The origin of the microlensing events observed towards the LMC and the stellar counterpart of the Magellanic stream

Abstract

We introduce a novel theoretical model to explain the longstanding puzzle of the nature of the microlensing events reported towards the Large Magellanic Cloud (LMC) by the massive compact halo object (MACHO) and Optical Gravitational Lensing Experiment (OGLE) collaborations. We propose that a population of tidally stripped stars from the Small Magellanic Cloud (SMC) located similar to 4-10 kpc behind a lensing population of LMC disc stars can naturally explain the observed event durations (17-71 d), event frequencies and spatial distribution of the reported events. Differences in the event frequencies reported by the OGLE (similar to 0.33 yr(-1)) and MACHO (similar to 1.75 yr(-1)) surveys appear to be naturally accounted for by their different detection efficiencies and sensitivity to faint sources. The presented models of the Magellanic System were constructed without prior consideration of the microlensing implications. These results favour a scenario for the interaction history of the Magellanic Clouds, wherein the Clouds are on their first infall towards the Milky Way and the SMC has recently collided with the LMC 100-300 Myr ago, leading to a large number of faint sources distributed non-uniformly behind the LMC disc. In contrast to self-lensing models, microlensing events are also expected to occur in fields off the LMC's stellar bar since the stellar debris is not expected to be concentrated in the bar region. This scenario leads to a number of observational tests: the sources are low-metallicity SMC stars; they exhibit high velocities relative to LMC disc stars that may be detectable via proper motion studies and, most notably, there should exist a stellar counterpart to the gaseous Magellanic Stream and Bridge with a V-band surface brightness of >34 mag arcsec(-2). In particular, the stellar Bridge should contain enough RR Lyrae stars to be detected by the ongoing OGLE survey of this region.