What galaxy masses perturb the local cosmic expansion?

TL;DR

This study uses cosmological simulations to explore how galaxy masses influence local cosmic expansion, revealing that significant mass lies outside virial radii and affects local flow measurements.

Contribution

It demonstrates that actual perturbing masses extend beyond virial radii and clarifies the relationship between different mass estimates in the local universe.

Findings

Mass outside virial radius significantly perturbs local Hubble flow.

Peculiar velocities provide unbiased virial mass ratios.

Outer halo profiles suggest total masses align with perturbing masses.

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 ($M_{\rm vir,1}$ 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. 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 $\rho\sim R^{-4}$ at $R\gtrsim R_{\rm vir}$, 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 $M_{\rm vir}$ based on the dynamics of tracers at $R\gg R_{\rm vir}$ 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.