Timing Mass of the Local Group

The classic model of the Local Group (LG) is that of two dominant constituents, the Milky Way and M31, first separating and then detaching from the Hubble flow, leading to a nearly radial-approaching orbit. This simple model has been confronted by new measurements of the three-dimensional M31 kinematics, by cosmological simulations, and by theoretical understanding of the impact of massive substructures, such as the Large Magellanic Cloud. This article explores the consequences of new observations and theory on the determination of the mass and dynamics of the LG. ▪ The M31 tangential velocity measurement and contribution from the cosmological constant both increase the implied timing mass of the LG to be ∼5 × 1012 M⊙. ▪ Timing mass estimates for the LG tend to be greater than the sum of the Milky Way and M31 halo masses, and greater than independent LG mass estimators. ▪ Precision future kinematics have the potential to explore the origin of this difference and shed light on dark matter in the LG, the origin of its angular momentum, and possibly even local values of cosmological parameters.