The removal of cusps from galaxy centres by stellar feedback in the early Universe

A cosmological paradoxAlthough the standard cosmological model describes the large-scale structure of the Universe successfully, it does less well on a smaller scale. For instance the model predicts a lower dark-matter density at the centre of nearby galaxies (a cusp or dip in density profile) than is seen in observations, which suggest that galactic dark matter haloes have a 'flat' central core. Mashchenko et al. resolve this paradox with numerical simulations showing that random bulk motions of gas in small primordial galaxies, driven by stellar feedback in the form of supernovae, could have caused this flattening of the central dark-matter cusp. Once removed, this cusp cannot be reintroduced during subsequent galactic mergers, so in the present Universe both small and large galaxies would have flat dark-matter core density profiles.