Field-level inference of the Local Group and its surrounding mass distribution

Abstract

I will present a Bayesian hierarchical field-level inference framework for the Local Group and its immediate cosmological environment. ΛCDM initial conditions are conditioned on observational constraints on the masses, relative position, and velocities of the Milky Way and M31 haloes, and on the surrounding velocity field traced by isolated galaxies within 5 Mpc. This yields statistically representative realizations of the local matter distribution that simultaneously reproduce the Local Group system and its surrounding flow. I will describe this multi-resolution inference framework and show that, within a standard ΛCDM cosmology, these data require a strongly flattened mass distribution around the Local Group, aligned with the Local Sheet and bordered by underdense regions above and below the plane. In this geometry, the quiet Hubble flow with respect to the Local Group is consistent with the dynamical mass of the Milky Way-M31 system, resolving a long-standing tension in spherical models and showing that light traces mass in our neighbourhood on these scales.