Can MACHOs Probe the Shape of the Galaxy Halo?

Abstract

We study the prospects for the current microlensing searches, which have recently discovered several candidates, to yield useful information about the flattening of the Galaxy dark matter halo. Models of HI warps and N-body simulations of galaxy formation suggest that disks commonly form in oblate halos with a tilt between the disk and halo symmetry axes. The microlensing optical depth for the Large Magellanic Cloud depends sensitively on the disk-halo tilt angle in the Milky Way. We conclude that a much larger spread in values for $τ(LMC)$ is consistent with rotation curve constraints than previously thought, and that the ratio $τ(SMC)/τ(LMC)$ of the optical depth to the Small and Large Magellanic clouds is not a clean test of halo flattening.