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

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 $\sigma \le 196$ pc. Since astrophysical effects produce no feasible analogues for the $1.62_{-0.49}^{+0.47}$ kpc offset recently observed in Abell 3827, the observational result is in tension with the collisionless cold dark matter model assumed in our simulations.

5 pages, 2 figures, accepted for publication in MNRAS Letters

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