Gravitational microlensing and the galactic halo

By means of extensive galactic modeling we study the implications of the MACHO, EROS, and OGLE microlensing results for the composition of the dark halo of the Galaxy as well as for other properties of the Galaxy. While microlensing toward the LMC most directly probes the halo MACHO mass fraction, microlensing toward the bulge is almost equally important as it strongly constrains the Galactic model. We take the Galaxy to be comprised of luminous and dark disk components, a bulge, and a dark halo consisting of both MACHO’s and cold dark matter with each component being described by several observationally motivated parameters. We pare down an initial model space of millions of galactic models to viable models, those which are consistent with the observational data, including rotation curve, local projected mass density, and microlensing rates toward the LMC and bulge. On the basis of a conservative, minimal set of observational constraints an all-MACHO halo cannot yet be excluded, although in most viable models of the Galaxy the halo MACHO fraction is between 0% and 30%, consistent with expectations for a universe whose primary component is cold dark matter. An all-MACHO halo is required to be light, and when data on the local escape velocity and satellite-galaxy proper motions, which probe the extent of the dark halo, are taken into account, models which have a high MACHO mass fraction are ruled out. We also explore the possibility that there are no MACHO’s in the halo. Finally, we point out several important tests that could definitively exclude an all-MACHO halo, e.g., evidence that the optical depth for microlensing is less than 1.5×${10}^{\mathrm{-}7}$ toward the LMC or greater than 3×${10}^{\mathrm{-}6}$ toward the bulge. © 1996 The American Physical Society.