What Are MACHOs? Interpreting LMC Microlensing
I discuss two hypotheses that might explain LMC microlensing: the Halo stellar remnant lensing hypothesis and the unvirialized LMC lensing hypothesis. I show that white dwarfs cannot contribute substantially to the cosmic baryon budget; they are strongly constrained by chemical evolution and background light measurements. Although there have been some claims of direct optical detections of white dwarfs in the Halo, I show how the full sample of direct optical searches for halo lenses do not support the Halo lens hypothesis.
N-body simulations suggest that the LMC may be naturally excited out of virial equilibrium by tidal forcing from the Milky Way. New measurements of LMC kinematics not only do not rule out the unvirialized LMC lensing hypothesis, but even moderately favor it (at 95% confidence).
I discuss future microlensing observations that will resolve this issue: high quality detailed measurements of microlensing events, exotic microlensing events, comparing different lines of sight, and statistics of the HR diagram. These techniques have all been applied, but are not yet successful owing to the relatively low quality and quantity of Magellanic cloud microlensing events discovered by ground based search teams. A space based search program involving the SNAP satellite will generate enough events and high quality events to break the degeneracy
KeywordsDark Matter Optical Depth White Dwarf Proper Motion Dark Matter Candidate
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