IAUS379: Dynamical Masses of Local Group Galaxies

The rather intuitive concept of 'galaxy mass' is an ill-defined quantity in cosmology. First, because in an expanding, close-to-homogeneous Universe collapsed structures do not show well-defined boundaries, and second because the availability of dynamical tracers becomes very scarce in the outskirts of dark matter haloes. In this talk I will provide an overview of the timing argument, which models the relative motion of massive substructures in an expanding Universe as a restricted 3-body system. I will show that this method returns masses that are systematically higher than the mass enclosed within the nominal virial radius of a galaxy, thus complicating a direct comparison with 'dynamical' masses derived from halo tracers. As an application, I will summarize recent attempts to measure simultaneously the masses of our Galaxy, the Large Magellanic Cloud and M31.