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 $2\times {10}^{-25}\text{gcmm}3$ to $10\times {10}^{-25}\text{gcmm}3$. 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, $9.2_{-3.1}^{+3.8}\times {10}^{-25}\text{gcmm}3$. 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.