The LMC Microlensing Events: Evidence for a Warped and Flaring Milky Way Disk?

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Abstract
The simplest interpretation of the microlensing events toward the Large Magellanic Cloud detected by the MACHO and EROS collaborations is that about one-third of the halo of our own Milky Way galaxy exists in the form of objects of around 0.5 M☉. There are grave problems with this interpretation. A normal stellar population of 0.5 M☉ stars should be visible. The other obvious candidate for the lenses is a population of white dwarfs. However, the precursor population must have polluted the interstellar medium with metals, in conflict with current population II abundances. Here we propose a more conventional, but at the moment more speculative, explanation. Some of the lenses are stars in the disk of the Milky Way. They lie along the line of sight to the LMC because of warping and flaring of the Galactic disk. Depending on its scale length and ellipticity, the disk's optical depth may lie anywhere between 0.2×10−7 and 0.9×10−7. Together with contributions from the LMC disk and bar and perhaps even intervening stellar contaminants, the total optical depth may match the data within the uncertainties. Microlensing toward the LMC may be telling us more about the distorted structure and stellar populations of the outer Milky Way disk than the composition of the dark halo.