What are MACHOs? Interpreting LMC microlensing

Abstract

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).