The offsets between galaxies and their dark matter in Lambda-CDM

TL;DR

This study uses hydrodynamical simulations to measure galaxy-dark matter offsets, finding most are very small and inconsistent with larger observed offsets, challenging the collisionless cold dark matter model.

Contribution

It provides the first detailed simulation-based analysis of galaxy-dark matter offsets, showing they are generally negligible and questioning recent observational claims.

Findings

Over 95% of galaxies have offsets smaller than 700 pc.

Offsets follow a Maxwellian distribution with σ ≤ 196 pc.

Observed large offsets in Abell 3827 are inconsistent with collisionless dark matter.

Abstract

We use the "Evolution and Assembly of GaLaxies and their Environments" ( EAGLE ) suite of hydrodynamical cosmological simulations to measure offsets between the centres of stellar and dark matter components of galaxies. We find that the vast majority (>95%) of the simulated galaxies display an offset smaller than the gravitational softening length of the simulations (Plummer-equivalent $\epsilon = 700$ pc), both for field galaxies and satellites in clusters and groups. We also find no systematic trailing or leading of the dark matter along a galaxy's direction of motion. The offsets are consistent with being randomly drawn from a Maxwellian distribution with $\sigma \leq 196$ pc. Since astrophysical effects produce no feasible analogues for the $1.62^{+0.47}_{-0.49}$ kpc offset recently observed in Abell 3827, the observational result is in tension with the collisionless cold dark matter model assumed in our simulations.