The Local Halo Density

For almost 20 years models of the Galaxy have included a dark halo responsible for supporting a substantial fraction of the local rotation velocity and a flat rotation curve at large distances. Estimates of the local halo density range from 2 × 10 -25 g cm -3 to 10 × 10 -25 g cm -3 . By careful modeling of the Galaxy, taking account of the evidence that dark halos are flattened and recent microlensing data, we arrive at a more quantitative estimate, 9.2 + 3.8 −3.1 × 10 -25 g cm -3 . Microlensing toward the LMC indicates that only a small fraction, less than ~30%, can be in the form of MACHOs, which is consistent with the idea that most of the halo consists of cold dark matter particles.