Gravitational Microlensing in the NGC 3314A-B Galaxy Pair

Determining the composition of the dark matter that dominates the masses of galaxies is an important unsolved problem, and the results of the MACHO Collaboration suggest that some of Milky Way's dark matter may be in the form of very old white dwarfs. However, some have argued that the excess of microlensing events seen by MACHO are due to a larger than expected microlensing rate for lens stars in the LMC itself or its tidal debris. We propose to address this question by detecting microlensing events in the line-of-sight galaxy pair NGC 3314 A & B. The large line-of-sight distance between these galaxies gives an optical depth that is 3-4 orders of magnitude larger than if the source stars and lenses were in the same galaxy, and the fact that the background galaxy is a spiral ensures that there will be a sufficient number of bright, non-variable source stars. Our proposed observations should have the sensitivity to detect microlensing by both ordinary stars and dark matter in NGC 3314A {the foreground galaxy}. If there are dark matter microlensing events to be found, they can be clearly distinguished from stellar microlensing events because they will occur outside the visible disk of NGC 3314A. If baryonic dark matter is detected in NGC 3314A, we will be able to map its radial density variation.