Locally Cold Flows from Large-Scale Structure

TL;DR

The paper explains that the observed cold Hubble flow around the Milky Way is due to local large-scale structure geometry, particularly walls, and not a cosmological problem, with implications for mass estimates.

Contribution

It demonstrates that local wall dynamics cause cold flows and super-Hubble expansions, providing a new interpretation of local velocity fields independent of cosmology.

Findings

The local Hubble flow dispersion is about 30 km/s, matching observations.

Local walls induce super-Hubble flows with Hubble constants between 0.77 and 1.13.

Ignoring wall effects can underestimate the Milky Way's mass by up to 30%.

Abstract

We show that the "cold" Hubble flow observed for galaxies around the Milky Way does not represent a problem in cosmology but is due to the particular geometry and dynamics of our local wall. The behavior of the perturbed Hubble flow around the Milky Way is the result of two main factors: at small scales (R < 1 Mpc) the inflow is dominated by the gravitational influence of the Milky Way. At large scales (R > 1 Mpc) the out flow reflects the expansion of our local wall which "cools down" the peculiar velocities. This is an intrinsic property of walls and is independent of cosmology. We find the dispersion of the local Hubble flow (1 < R < 3 Mpc) around simulated "Milky Way" haloes located at the centre of low-density cosmological walls to be {\sigma}_H ~ 30 km/s, in excellent agreement with observations. The expansion of our local wall is also reflected in the value of the measured local Hubble constant. For "Milky Way" haloes inside walls, we find super-Hubble flows with h_local \simeq 0.77 - 1.13. The radius of equilibrium (R_0) depends not only on the mass of the central halo and the Hubble expansion but also on the dynamics given by the local LSS geometry. The super-Hubble flow inside our local wall has the effect of reducing the radius at which the local expansion balances the gravitational influence of the Milky Way. By ignoring the dynamical effect of the local wall, the mass of the Milky Way estimated from R_0 can be underestimated by as much as ~ 30%.