Asymmetries in the inner regions of LCDM haloes

Abstract

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 LCDM 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% of cluster halos have density centre separated from barycentre by more than 20% of the virial radius, while only 7% of Milky Way halos have such large asymmetries. About 40% of all cluster halos 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% of Milky Way halos have such large velocity offsets. About 25% of all cluster haloes have acquired more than a quarter of the mass currently in their inner 10kpc through mergers since z=1. The corresponding percentage of Milky Way haloes is 15%. 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.