Locally cold flows from large-scale structure

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 behaviour of theperturbedHubble flow around the Milky Way is the result of two main factors: at small scales (R< 1Mpc) theinflow is dominated by the gravitational influence of the Milky Way and at large scales (R> 1Mpc) theoutflow 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 < 3Mpc) around simulated 'Milky Way' haloes located at the centre of low-density cosmological walls to be σH~ 30kms-1, in excellent agreement with observations. The expansion of our local wall is also reflected in the value of the measuredlocalHubble constant. For 'Milky Way' haloes inside walls, we find super-Hubble flows withhlocal≃ 0.77-1.13 over a range of increasingly 'clean' environments. The radius of equilibrium (R0) 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 fromR0 can be underestimated by as much as ~30 per cent. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.