The mass distribution in and around the Local Group

Abstract

Our Galaxy, Andromeda and their companion dwarf galaxies form the Local Group. Most of the mass in and around it is believed to be dark matter rather than gas or stars, so its distribution must be inferred from the effect of gravity on the motion of visible objects. Modelling efforts have long struggled to reproduce the quiet Hubble flow around the Local Group, as they require unrealistically little mass beyond the haloes of the two main galaxies. Here we revisit this using $\Lambda$CDM simulations of Local Group analogues with initial conditions constrained to match the observed dynamics of the two main haloes and the surrounding flow. The observations are reconcilable within $\Lambda$CDM, but only if mass is strongly concentrated in a plane out to 10 Mpc, with the surface density rising away from the Local Group and with deep voids above and below. This configuration, dynamically inferred, mirrors known structures in the nearby galaxy distribution. The resulting Hubble flow is quiet yet strongly anisotropic, a fact obscured by the paucity of tracers at high supergalactic latitude. This flattened geometry reconciles the dynamical mass estimates of the Local Group with the surrounding velocity field, thus demonstrating full consistency within the standard cosmological model.

Published in Nature Astronomy

References in corpus (22)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Cosmicflows-3
• Simulating cosmic structure formation with the GADGET-4 code
• Our Peculiar Motion Away from the Local Void
• Bayesian physical reconstruction of initial conditions from large scale structure surveys
• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• Masses for the Local Group and the Milky Way
• Clues from nearby galaxies to a better theory of cosmic evolution
• Physical Bayesian modelling of the non-linear matter distribution: new insights into the Nearby Universe
• The very local Hubble flow
• Local galaxy flows within 5 Mpc
• A dynamical model of the local cosmic expansion
• Radial and Transverse Velocities of Nearby Galaxies
• An exact general remeshing scheme applied to physically conservative voxelization
• A Council of Giants
• The Sheet of Giants: Unusual Properties of the Milky Way's Immediate Neighbourhood
• All-Sky Kinematics of the Distant Halo: The Reflex Response to the LMC
• Constrained cosmological simulations of the Local Group using Bayesian hierarchical field-level inference
• The effect of environment on the mass assembly history of the Milky Way and M31