Searching for MACHOs (and other dark matter candidates) in a simulated galaxy

We conduct gravitational microlensing experiments in a galaxy taken from a cosmological N-body simulation. Hypothetical observers measure the optical depth and event rate toward hypothetical LMCs and compare their results wit h model predictions. Because we control the accuracy and sophistication of the model, we can determine how good it has to be for statistical errors to dominate over systematic ones. Several thousand independent microlensing e xperiments are performed. When the "best-fit" triaxial model for the mass d istribution of the halo is used, the agreement between the measured and pre dicted optical depths is quite good: by and large, the discrepancies are co nsistent with statistical fluctuations. If on the other hand, a spherical m odel is used, systematic errors dominate. Even with our "best-fit" model, t here are a few rare experiments where the deviation between the measured an d predicted optical depths cannot be understood in terms of statistical flu ctuations. In these experiments there is typically a clump of particles cro ssing the line of sight to the hypothetical LMC. These clumps can be either gravitationally bound systems or transient phenomena in a galaxy that is s till undergoing phase mixing. Substructure of this type, if present in the Galactic distribution of MACHOs, can lead to large systematic errors in the analysis of microlensing experiments. We also describe how hypothetical WI MP and axion detection experiments might be conducted in a simulated N-body galaxy.