What galaxy masses perturb the local cosmic expansion?

Understand

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 ($M_{\rm vir,1}$ and $M_{\rm vir,2}$), the mass derived from the relative motion of the halo pair ($M_{\rm tim}$), and that inferred from the local Hubble flow ($M_{\rm lhf}$).

• We show that within the Spherical Collapse Model (SCM), the correspondence between the three mass estimates is exact, i.e.
• $M_{\rm lhf}=M_{\rm tim}=M_{\rm vir,1}+M_{\rm vir,2}$.
• 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 $R_{\rm vir}$, and that the amount of "extra-virial" mass tends to increase in galaxies with a slow accretion rate.