Are great disks defined by satellite galaxies in Milky-Way type halos rare in ΛCDM?

We study the spatial distribution of satellite galaxies by assuming that they follow the dark matter distribution. This assumption is supported by semi- analytical studies based on high- resolution numerical simulations. We find that for a Milky- Way type halo, if only a dozen satellite galaxies are observed, then they can lie on a great disk with an rms height of about 40 kpc. The normal to the plane is roughly isotropic on the sky. These results are consistent with the observed properties of the satellite galaxies in the Milky Way. If, however, the satellite galaxies follow the distribution of substructure selected by the present mass, then great disks similar to the one in the Milky Way will be rare and difficult to reproduce, in agreement with the conclusion reached by Kroupa et al. ( 2004).