A dynamical model of the local cosmic expansion

We combine the equations of motion that govern the dynamics of galaxies in the local volume with Bayesian techniques in order to fit orbits to published distances and velocities of galaxies within 3 Mpc. We find a Local Group (LG) mass 2.3 \ub1 0.7 7 10^12 M 99 that is consistent with the combined dynamical masses of M31 and the Milky Way, and a mass ratio 0.54+0.23 -0.17 that rules out models where our Galaxy is more massive than M31 with ~95 per cent confidence. The MilkyWay's circular velocity at the solar radius is relatively high, 245\ub123 km s-1, which helps to reconcile the mass derived from the local Hubble flow with the larger value suggested by the 'timing argument'. Adopting Planck's bounds on \u3a9\u39b yields a (local) Hubble constant H0 = 67 \ub1 5 kms-1 Mpc-1 which is consistent with the value found on cosmological scales. Restricted N-body experiments show that substructures tend to fall on to the LG along the Milky Way-M31 axis, where the quadrupole attraction is maximum. Tests against mock data indicate that neglecting this effect slightly overestimates the LG mass without biasing the rest of model parameters.We also show that both the time dependence of the LG potential and the cosmological constant have little impact on the observed local Hubble flow.Peer reviewed: YesNRC publication: Ye