New Mass Estimators for Tracer Populations

We introduce the tracer mass estimator. This is a new and simple way to estimate the enclosed mass from the projected positions and line of sight velocities of a tracer population (such as globular clusters, halo stars and planetary nebulae). Like the projected mass estimator, it works by averaging (projected distance) x (radial velocity squared)/ G over the sample. However, it applies to the commonest case of all, when the tracer population does not follow the overall dark matter density. The method is verified against simulated datasets drawn from Monte Carlo realisations of exact solutions of the collisionless Boltzmann equation and applied to the recent M31 globular cluster data set of Perrett et al. (2002), as well as to M31's satellite galaxies.