The Local Halo Density

For almost twenty 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 $2\times 10^{-25}\gcmm3$ to $10\times 10^{-25}\gcmm3$. 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, $9.2^{+3.8}_{-3.1}\times 10^{-25}\gcmm3$. Microlensing toward the LMC indicates that only a small fraction, less than about 30\%, can be in the form of MACHOs, consistent with the idea that most of the halo consists of cold dark matter particles.