What galaxy masses perturb the local cosmic expansion?

Abstract We use 12 cosmological N-body simulations of Local Group systems (the apostle models) to inspect the relation between the virial mass of the main haloes (Mvir,1 and Mvir,2), the mass derived from the relative motion of the halo pair (Mtim), and that inferred from the local Hubble flow (Mlhf). We show that within the spherical collapse model (SCM), the correspondence between the three mass estimates is exact, i.e. Mlhf = Mtim = Mvir,1 + Mvir,2. However, comparison with apostle simulations reveals that, contrary to what the SCM states, a relatively large fraction of the mass that perturbs the local Hubble flow and drives the relative trajectory of the main galaxies is not contained within Rvir, and that the amount of ‘extravirial’ mass tends to increase in galaxies with a slow accretion rate. In contrast, modelling the peculiar velocities around the Local Group returns an unbiased constraint on the virial mass ratio of the main galaxy pair. Adopting the outer halo profile found in N-body simulations, which scales as ρ ∼ R−4 at R ≳ Rvir, indicates that the galaxy masses perturbing the local Hubble flow roughly correspond to the asymptotically convergent (total) masses of the individual haloes. We show that estimates of Mvir based on the dynamics of tracers at R ≫ Rvir require a priori information on the internal matter distribution and the growth rate of the main galaxies, both of which are typically difficult to quantify.