http://arxiv.org/abs/1609.07147
We use 12 cosmological $N$-body simulations of Local Group systems (the Apostle models; Sawala et al. 2016) 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), which provides an idealized description of structure formation in an expanding Universe, 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 addition, we find that modelling the peculiar velocities around the Local Group returns an unbiased constraint on the virial mass ratio of the main galaxy pair ($f_m\equiv M_1/M_2\approx M_{\rm vir,1}/M_{\rm vir,2}$), as well as the individual masses of the main galaxies ($M_1$ and $M_2$) without a priori assumptions on the matter distribution nor the equilibrium state of these systems. Adopting Diemer \& Kravtsov (2014) outer halo profile, which scales as $\rho\sim R^{-4}$ at $R\gtrsim R_{\rm vir}$, indicates that $M_1$ and $M_2$ roughly correspond to the asymptotically-convergent (total) masses of the individual haloes. In contrast, we find 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.
J. Penarrubia and A. Fattahi
Mon, 26 Sep 16
20/48
Comments: 17 pages, 9 figures, 2 tables. Submitted to MNRAS
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