Asymmetries in the inner regions of ΛCDM haloes

Many galaxies display warps, lopsided images, asymmetric rotation curves or other features which suggest that their immediate dynamical environment is neither static nor in equilibrium. In cold dark matter (CDM) theories, such non-equilibrium features are expected in the inner regions of many dark haloes as a result of recent hierarchical growth. We used the excellent statistics provided by the very large Millennium Simulation to study (i) how the distribution of position and velocity asymmetries predicted for halo cores by the concordance ACDM cosmogony depends on halo mass and (ii) how much of the dark matter in the inner core has been added at relatively recent times. Asymmetries are typically larger in more massive haloes. Thus 20 per cent of cluster haloes have density centre separated from barycentre by more than 20 per cent of the virial radius, while only 7 per cent of Milky Way haloes have such large asymmetries. About 40 per cent of all cluster haloes have a mean core velocity which differs from the barycentre velocity by more than a quarter of the characteristic halo circular velocity, whereas only 10 per cent of Milky Way haloes have such large velocity offsets. About 25 per cent of all cluster haloes have acquired more than a quarter of the mass currently in their inner 10 kpc through mergers since z = 1. The corresponding percentage of Milky Way haloes is 15 per cent. These numbers seem quite compatible with the levels of asymmetry seen in the observable regions of galaxies, but quantitative comparison requires more detailed modelling of the observable components.