The spatial distribution of galactic satellites in the Λ cold dark matter cosmology

We investigate the spatial distribution of galactic satellites in high-resolution simulations of structure formation in the cold dark matter ( CDM) model: the Aquarius dark matter simulations of individual haloes and the Millennium-II simulation of a large cosmological volume. To relate the simulations to observations of the Milky Way we use two alternative models to populate dark haloes with 'visible' galaxies: a semi-analytic model of galaxy formation and an abundance matching technique. We find that the radial density profile of massive satellites roughly follows that of the dark matter halo (unlike the distribution of dark matter subhaloes). Furthermore, our two galaxy formation models give results consistent with the observed profile of the 11 classical satellites of the Milky Way. Our simulations predict that larger, fainter samples of satellites should still retain this profile at least up to samples of 100 satellites. The angular distribution of the classical satellites of the Milky Way is known to be highly anisotropic. Depending on the exact measure of flattening, 5-10 per cent of satellite systems in our simulations are as flat as the Milky Way's and this fraction does not change when we correct for possible obscuration of satellites by the Galactic disc. A moderate flattening of satellite systems is a general property of CDM, best understood as the consequence of preferential accretion along filaments of the cosmic web. Accretion of a single rich group of satellites can enhance the flattening due to such anisotropic accretion. We verify that a typical Milky Way-mass cold dark matter halo does not acquire its 11 most massive satellites from a small number of rich groups. Single-group accretion becomes more likely for less massive satellites. Our model predictions should be testable with forthcoming studies of satellite systems in other galaxies and surveys of fainter satellites in the Milky Way.