The phase-space structure of cold dark matter haloes: insights into the Galactic halo

We study the formation of the halo of the Milky Way in a -cold dark matter ( CDM) cosmology by scaling down a high-resolution simulation of the formation of a cluster of galaxies. We determine how much phase-space substructure is left over from the objects that merge to build up the present galaxy. We study the debris streams originating from such objects and find that their evolution can be explained simply in terms of the conservation of phasespace density. Analysing the mass growth history of our halo we find that its inner regions have been in place for more than 10 Gyr, but that the growth of the halo as a whole is more gradual, in agreement with other high-resolution simulations of dark matter haloes. Recent accretion contributes to the inner 10 kpc of the halo only at the 10 -4 level. Finally, we determine the number of dark matter streams as a function of distance from the centre of the halo. In the equivalent of the 'solar vicinity', we find that the dark matter is smoothly distributed in space, and that the velocity ellipsoid is formed by hundreds of thousands of streams, most of which have velocity dispersions of the order of 1 km s -1 or less.