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 x 10(-25) g cm(-3) to 10 X 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(-3.1)(+3.8) X 10(-25) g cm(-3). Microlensing toward the LMC indicates that only a small fraction, less than similar to 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.